perf: Pre-compute hugr capacity when decoding a hugr-model

[aborgna-q]

Analyse the model before allocating the hugr, so we can pass approximate capacities for its internal vectors and avoid vector reallocations during decoding.

[codecov]

Codecov Report

❌ Patch coverage is 94.17476% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.58%. Comparing base (3aa55e9) to head (2bdead3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	94.17%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3219      +/-   ##
==========================================
+ Coverage   81.55%   81.58%   +0.02%     
==========================================
  Files         242      242              
  Lines       47309    47408      +99     
  Branches    40906    41005      +99     
==========================================
+ Hits        38584    38677      +93     
- Misses       6713     6716       +3     
- Partials     2012     2015       +3     

Flag	Coverage Δ
python	89.39% <ø> (ø)
rust	80.36% <94.17%> (+0.03%)	⬆️

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[codspeed-hq]

Merging this PR will improve performance by 14.17%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
✅ 43 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	serialization/big_hugr/1048576/capnp/with_extensions/encode	17.6 ms	15.4 ms	+14.17%
⚡	serialization/big_hugr/1048576/capnp/without_extensions/encode	17.6 ms	15.4 ms	+14.16%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ab/perf-model-import-capacity (2bdead3) with main (3aa55e9)}