T1.2.1: Add rustdoc on MediaType variants and methods

docs/PRD/reports/T1.2.1-report.md

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+# T1.2.1 — Add rustdoc on `MediaType` variants and methods
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-11T07:23Z
+**Completed:** 2026-05-11T07:24Z
+**Commit:** see git log
+**PRD:** ../PRD-wire-format-v2.md
+
+## What I changed
+
+- `crates/wzp-proto/src/media_type.rs:7-10` — added `///` doc comments to all 4 variants (`Audio`, `Video`, `Data`, `Control`)
+- `crates/wzp-proto/src/media_type.rs:14` — added `///` doc on `to_wire`
+- `crates/wzp-proto/src/media_type.rs:18` — added `///` doc on `from_wire`
+
+## Why these choices
+
+Follow-up from T1.2 review: coding standard #9 requires `///` on all public items. Added concise one-line docs matching the examples from the task spec.
+
+## Deviations from the task spec
+
+None.
+
+## Verification output
+
+```bash
+$ cargo doc -p wzp-proto --no-deps 2>&1 | grep -i "missing" || echo "no missing-doc warnings"
+no missing-doc warnings
+```
+
+```bash
+$ cargo clippy -p wzp-proto --all-targets -- -D warnings -W missing_docs 2>&1 | grep "media_type.rs:" | head -10
+# (no output — no missing-docs in media_type.rs)
+```
+
+```bash
+$ cargo build -p wzp-proto
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.49s
+```
+
+```bash
+$ cargo test -p wzp-proto --no-fail-fast
+running 112 tests
+test result: ok. 112 passed; 0 failed; ...
+```
+
+```bash
+$ cargo fmt --all -- --check
+# (clean)
+```
+
+## Test summary
+
+- Tests added: 0
+- Tests modified: 0
+- `cargo clippy -p wzp-proto --all-targets -- -D warnings`: pass
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+None.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real (re-run if suspicious)
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved