[Routine - MIGAO] Add unit test coverage for PinyinDict TSV parsing

[winterdrive]

Pre-flight Check

• Open PRs at time of scan: #37 "[Routine - MIGAO] Add unit test coverage for tray tooltip truncate() helper" — touches src/bin/watch.rs only.
• Active routine branches: routine/migao-fix-tray-truncate-test-coverage-260819 (= PR [Routine - MIGAO] Add unit test coverage for tray tooltip truncate() helper #37, src/bin/watch.rs), routine/migao-fix-rule-apply-top-n-coverage-260818 (already merged as PR [Routine - MIGAO] Add unit test coverage for Rule trait default apply_top_n #27, touched src/rule.rs).
• This PR touches only src/pinyin_dict.rs, which is untouched by PR [Routine - MIGAO] Add unit test coverage for tray tooltip truncate() helper #37 and by any active branch — no overlap.

Changes

src/pinyin_dict.rs had zero test coverage. Extracted the inline TSV-parsing loop in PinyinDict::load() into a standalone parse_tsv_into() function (mirroring the existing pattern already used in src/dict.rs), and added unit tests covering:

• blank lines and # comment lines are skipped
• malformed rows (wrong tab-field count) are skipped
• an unparseable frequency column falls back to 1
• multiple entries under the same pinyin key accumulate correctly
• global() loads a non-empty dict from the embedded data/pinyin.tsv

This PR does not modify core dictionary data (data/bopomofo.tsv or any embedded RIME resources), does not add/remove/update any Cargo.toml dependencies, and does not touch package version metadata.

Safety Verification

Run locally on this branch, all passed:

• cargo check --all-targets — passed
• cargo clippy --all-targets -- -D warnings — passed, no warnings
• cargo test — 101 passed (up from 96), 0 failed, in the migao lib test binary; migao-watch bin tests (6) and doc-tests unaffected
• cargo fmt --check — passed, no diff

CI / Release Gate Note

This is a daily routine Draft PR. Package version bump (Cargo.toml/Cargo.lock) and CHANGELOG.md consolidation are intentionally deferred to the weekend release/integration PR.