perf: Cache serialized extensions to speed up encoding

[aborgna-q]

Keep the serialized json buffer around when loading an extension in Rust, so we don't need to re-encode them each time.

Extensions are shared inside Arcs, so the buffer is only stored once.

[codecov]

Codecov Report

❌ Patch coverage is 73.41772% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.55%. Comparing base (3aa55e9) to head (d98ef26).

Files with missing lines	Patch %	Lines
hugr-core/src/extension.rs	73.68%	8 Missing and 7 partials ⚠️
hugr-core/src/envelope/reader.rs	77.77%	0 Missing and 4 partials ⚠️
hugr-core/src/envelope/writer.rs	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3210      +/-   ##
==========================================
- Coverage   81.55%   81.55%   -0.01%     
==========================================
  Files         242      242              
  Lines       47309    47373      +64     
  Branches    40906    40970      +64     
==========================================
+ Hits        38584    38633      +49     
- Misses       6713     6721       +8     
- Partials     2012     2019       +7     

Flag	Coverage Δ
python	89.39% <ø> (ø)
rust	80.32% <73.41%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will improve performance by 32.63%

⚡ 3 improved benchmarks
✅ 42 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	serialization/simple_cfg/capnp/with_extensions/encode	490.1 µs	325 µs	+50.78%
⚡	serialization/t_factory/capnp/with_extensions/encode	14.6 ms	11.1 ms	+31.09%
⚡	serialization/simple_cfg/sexpr/with_extensions/encode	1.2 ms	1 ms	+18.05%

Tip

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

---

_{Comparing ab/extension-serialization-cache (d98ef26) with main (3aa55e9)}