[walterjgsp]

Summary

Phase 2 of #46, Rust. Each rust_test keeps the llvm-cov report as lcov, with the function (FN/FNDA) and branch (BRDA) records that Please's merged coverage.xml / coverage.json drop:

• Declared as a test output: plz-out/bin/<package>/<name>.lcov, repository-relative paths, toolchain and registry sources excluded. Uses the shared tools/common/lcov package (feat(common): add shared lcov package #47).
• Partial lines: stable rustc emits no branch regions, so BRDA records are synthesized from llvm-cov export -format=text sub-line regions with lcov.AddRegionBranches (feat(common): derive partial-line branches from llvm-cov regions #50). Only lines that ran but contain a never-run sub-line region get one (e.g. the right side of a && b). If the JSON export fails the report is still written, without them.
• Created empty up front, so it exists (and is empty) without coverage; the report Please reads is unchanged.
• Fix found on the way: under Please TMP_DIR is the working directory, so the raw profiles were listed twice and merged twice, doubling every execution count. Line coverage was unaffected; the exported counts would have been wrong. They are now listed once (test added).
• New please_rust test-runner --lcov-file flag; docs; changelog. The [Unreleased] section (LLVM tool fix, docs) becomes 0.6.0 together with this new feature; VERSION is bumped by the release workflow.

Heuristic limits (documented)

A multi-line if/else is not a partial line: the untaken side shows up as an uncovered line. FN names are rustc v0-mangled.

Test plan

• plz test //... (96 tests): export built from real llvm-cov lcov and JSON (toolchain file dropped, paths normalized, line 12 partial, uncalled function uncovered), no-JSON fallback, invalid input, empty reset, single listing of profiles
• New fixture test/rust/branches:branches_test: under plz cover FNDA:2 for classify and FNDA:0 for unused (counts correct after the profile fix), BRDA:12,0,0,0; a plain plz test leaves the file empty
• CI smoke step asserting those records

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