fix: sort response headers for deterministic order

[kitsuyui]

Summary

Response headers forwarded by the proxy were enumerated in a non-deterministic
order. Playwright's response.headers() returns a plain object whose key
ordering depends on the browser engine and the upstream server's transmission
order. As a result, identical requests could produce different header orderings
across browser types or runs.

This change adds a sortHeaders utility that sorts header keys
lexicographically and applies it in renderToResponse after appendVaryHeader,
so the proxy always emits headers in a stable alphabetical order regardless of
the upstream or browser source.

Changes

• src/lib/headers.ts: add and export sortHeaders(headers: Headers): Headers
• src/lib/headers.spec.ts: unit tests for sortHeaders (empty input,
  lexicographic order, value preservation, input immutability)
• src/server/index.ts: wrap appendVaryHeader(...) result with sortHeaders
  in renderToResponse

Verification

• bun run lint: clean
• bun run typecheck: clean
• bun run vitest run src/lib/headers.spec.ts: 15/15 passed
• collateral check: no violations

Trade-offs

Sorting adds one O(n log n) pass over the header set per response, where n is
the number of forwarded headers (typically < 20). The overhead is negligible
compared to the browser render time.

[github-actions]

gh-counter

PR gate

	Removed	Added	+/-
TODO/expect-error	0	0	0

Repo dashboard

	main (de5f702)	#1646 (b9cc2fe)	+/-
TODO/expect-error	4	4	0

---

Reported by gh-counter

[github-actions]

Code Metrics Report

	main (de5f702)	#1646 (b9cc2fe)	+/-
Coverage	86.9%	87.2%	+0.2%
Code to Test Ratio	1:0.5	1:0.5	+0.0
Test Execution Time	6s	7s	+1s

Details

  |                     | main (de5f702) | #1646 (b9cc2fe) |  +/-  |
  |---------------------|----------------|-----------------|-------|
+ | Coverage            |          86.9% |           87.2% | +0.2% |
  |   Files             |             11 |              11 |     0 |
  |   Lines             |            207 |             211 |    +4 |
+ |   Covered           |            180 |             184 |    +4 |
+ | Code to Test Ratio  |          1:0.5 |           1:0.5 |  +0.0 |
  |   Code              |           1648 |            1694 |   +46 |
+ |   Test              |            961 |             994 |   +33 |
- | Test Execution Time |             6s |              7s |   +1s |

Code coverage of files in pull request scope (86.0% → 86.6%)

Files	Coverage	+/-	Status
src/lib/headers.ts	100.0%	0.0%	modified
src/server/index.ts	76.9%	0.0%	modified

---

Reported by octocov