perf: avoid cloning record fields during decode

[sasa-tomic]

Overview
Reduce per-record allocation churn during Candid record decoding.

Requirements
Preserve existing wire compatibility and keep record decoding behavior unchanged.

Solution
Track expected and wire record fields by type plus index instead of cloning field queues for each decoded record. Add a compatibility test covering backward and forward compatible record vectors.

Considerations
This keeps the wire format unchanged and is intended to remain fully compatible with existing Candid data. Series-level benchmark context is tracked in #710.

[github-actions]

Name	Max Mem (Kb)	Encode	Decode
blob	4_224	4_207_487	2_122_432
btreemap	73_856	531_975_925 ($\textcolor{green}{-0.00\%}$)	12_984_691_960 ($\textcolor{green}{-0.43\%}$)
nns	192	2_021_253	5_663_909 ($\textcolor{green}{-0.90\%}$)
nns_list_proposal	1_216	7_017_181 ($\textcolor{red}{0.09\%}$)	64_183_554 ($\textcolor{green}{-5.36\%}$)
option_list	64 ($\textcolor{green}{-50.00\%}$)	716_007 ($\textcolor{green}{-0.00\%}$)	21_855_613 ($\textcolor{green}{-7.19\%}$)
text	6_336	4_204_384	7_877_759
variant_list	64 ($\textcolor{green}{-50.00\%}$)	710_989 ($\textcolor{green}{-0.01\%}$)	20_436_541 ($\textcolor{green}{-8.09\%}$)
vec_int16	16_704	123_694_298	998_268_455 ($\textcolor{green}{-1.65\%}$)

• Parser cost: 17_069_949
• Extra args: 2_851_812 ($\textcolor{green}{-16.53\%}$)

Click to see raw report

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Benchmark: blob
  total:
    instructions: 6.33 M (no change)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 4.21 M (no change)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 2.12 M (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: btreemap
  total:
    instructions: 13.52 B (-0.41%) (change within noise threshold)
    heap_increase: 1154 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 531.98 M (-0.00%) (change within noise threshold)
    heap_increase: 159 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 12.98 B (-0.43%) (change within noise threshold)
    heap_increase: 995 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: extra_args
  total:
    instructions: 2.85 M (improved by 16.53%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: nns
  total:
    instructions: 25.59 M (-0.20%) (change within noise threshold)
    heap_increase: 3 pages (no change)
    stable_memory_increase: 0 pages (no change)

  0. Parsing (scope):
    calls: 1 (no change)
    instructions: 17.07 M (no change)
    heap_increase: 3 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 2.02 M (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 5.66 M (-0.90%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: nns_list_proposal
  total:
    instructions: 71.20 M (improved by 4.85%)
    heap_increase: 19 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 7.02 M (0.09%) (change within noise threshold)
    heap_increase: 5 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 64.18 M (improved by 5.36%)
    heap_increase: 14 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: option_list
  total:
    instructions: 22.57 M (improved by 6.97%)
    heap_increase: 1 pages (improved by 50.00%)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 716.01 K (-0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 21.86 M (improved by 7.19%)
    heap_increase: 1 pages (improved by 50.00%)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: text
  total:
    instructions: 12.08 M (no change)
    heap_increase: 99 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 4.20 M (no change)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 7.88 M (no change)
    heap_increase: 33 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: variant_list
  total:
    instructions: 21.15 M (improved by 7.84%)
    heap_increase: 1 pages (improved by 50.00%)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 710.99 K (-0.01%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 20.44 M (improved by 8.09%)
    heap_increase: 1 pages (improved by 50.00%)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: vec_int16
  total:
    instructions: 1.12 B (-1.47%) (change within noise threshold)
    heap_increase: 261 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    calls: 1 (no change)
    instructions: 123.69 M (no change)
    heap_increase: 261 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    calls: 1 (no change)
    instructions: 998.27 M (-1.65%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Summary:
  instructions:
    status:   Improvements detected 🟢
    counts:   [total 9 | regressed 0 | improved 4 | new 0 | unchanged 5]
    change:   [max 0 | p75 -52.20K | median -1.69M | p25 -3.63M | min -55.57M]
    change %: [max 0.00% | p75 -0.20% | median -1.47% | p25 -6.97% | min -16.53%]

  heap_increase:
    status:   Improvements detected 🟢
    counts:   [total 9 | regressed 0 | improved 2 | new 0 | unchanged 7]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min -1]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min -50.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 9 | regressed 0 | improved 0 | new 0 | unchanged 9]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------

Only significant changes:
| status | name                           | calls |    ins |  ins Δ% | HI |   HI Δ% | SMI |  SMI Δ% |
|--------|--------------------------------|-------|--------|---------|----|---------|-----|---------|
|   -    | nns_list_proposal              |       | 71.20M |  -4.85% | 19 |   0.00% |   0 |   0.00% |
|   -    | nns_list_proposal::2. Decoding |     1 | 64.18M |  -5.36% | 14 |   0.00% |   0 |   0.00% |
|   -    | option_list                    |       | 22.57M |  -6.97% |  1 | -50.00% |   0 |   0.00% |
|   -    | option_list::2. Decoding       |     1 | 21.86M |  -7.19% |  1 | -50.00% |   0 |   0.00% |
|   -    | variant_list                   |       | 21.15M |  -7.84% |  1 | -50.00% |   0 |   0.00% |
|   -    | variant_list::2. Decoding      |     1 | 20.44M |  -8.09% |  1 | -50.00% |   0 |   0.00% |
|   -    | extra_args                     |       |  2.85M | -16.53% |  0 |   0.00% |   0 |   0.00% |

ins = instructions, HI = heap_increase, SMI = stable_memory_increase, Δ% = percent change

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Successfully persisted results to canbench_results.yml