feat: add --compression flag (default off) to control Accept-Encoding

[paveg]

Summary

• Add opt-in --compression CLI flag (default off).
• When off, inject Accept-Encoding: identity so response_size_bytes reflects wire bytes, matching pre-0.1.8 semantics for compressible targets.
• When on, fall through to async 0.17.1's default gzip handling for realistic benchmarking of compressed endpoints.
• User-provided Accept-Encoding via -H is honored and never overwritten.

Rationale

molt 0.1.8 pulled in moonbitlang/async 0.17.1, which started sending Accept-Encoding: gzip by default and transparently decompressing responses. That shifted response_size_bytes from wire bytes to decompressed payload size — a surprising regression for users who care about bandwidth measurement.

The behavior matches curl (which does not send Accept-Encoding unless you pass --compressed). It's an opt-in rather than opt-out because wire-byte measurement is the conservative default for a load testing tool.

Implementation

• New compression : Bool field on Config (default false).
• --compression wired through src/cmd/main/cli.mbt.
• build_headers in src/lib/worker/worker.mbt injects Accept-Encoding: identity when compression=false and the user hasn't already set their own.
• Debug path in src/cmd/main/main.mbt applies the same logic.
• All Config literal construction sites updated.

Test plan

• Whitebox tests for build_headers covering: compression off (injects identity), compression on (passthrough), user-provided Accept-Encoding preserved.
• moon check --target native clean
• moon fmt clean
• moon test --target native — all tests pass (250 -> 254)
• moon build --target native --release — binary builds

Closes #14

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.46%. Comparing base (75361f8) to head (8e77f7b).

Files with missing lines	Patch %	Lines
src/cmd/main/main.mbt	0.00%	2 Missing ⚠️
src/lib/worker/worker.mbt	66.66%	2 Missing ⚠️
src/cmd/main/cli.mbt	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   62.48%   62.46%   -0.02%     
==========================================
  Files          16       16              
  Lines         733      738       +5     
==========================================
+ Hits          458      461       +3     
- Misses        275      277       +2     

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