Fix mutation baseline by making heuristics::text doctests compile

[leynos]

Closes #66.

Three doctests in src/heuristics/text.rs imported items via lag_complexity::heuristics::text, but the module was declared pub(crate), so plain cargo test --all-features failed with E0603 while CI stayed green (make test passed --all-targets, which skips doctests). The scheduled cargo-mutants workflow validates its unmutated baseline with plain cargo test --all-features, so the baseline failed before any mutants ran.

Changes

• src/heuristics/mod.rs: promote heuristics::text from pub(crate) to pub. The module's functions were already pub, #[must_use]-annotated, and documented with examples written against the public path, so publishing the module keeps the examples as real, runnable doctests rather than demoting them to ignore prose. The issue lists this as an acceptable resolution, and the crate is pre-1.0.
• src/heuristics/text.rs: add #[must_use] to weighted_count, satisfying the pedantic must_use_candidate lint now that the function is externally visible.
• Makefile: the test target now also runs cargo test --doc --all-features, closing the blind spot permanently — --all-targets will never again mask a doctest that plain cargo test (as run by cargo-mutants) compiles.

Validation

• cargo test --all-features (doctests included): three consecutive green runs; all 18 doctests pass, including the three previously failing ones (normalize_tokens, weighted_count, substring_count_regex).
• make test (including the new doctest step), make typecheck, make check-fmt, make markdownlint: green.
• make lint: Clippy green. The locally installed Whitaker suite reports pre-existing unwrap_or_else findings in tests/ambiguity_heuristic.rs and tests/lagc_args.rs — files untouched by this diff; CI pins whitaker-installer 0.2.5 and is green on main, so these are a local toolchain-vintage discrepancy, not introduced here.
• The issue Flaky proptest: possessive_proper_noun_anchors_pronoun fails on generated input (name=Alice, pronoun=It, verb=her) #68 proptest flake did not trip during any of the runs.

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