feat: add Rust path remap auto support

[zackees]

Closes #229.

Summary

• add ZCCACHE_PATH_REMAP=auto support for rustc/clippy by injecting a root-covering --remap-path-prefix <worktree-root>=. when needed
• gate Rust cross-root request/context sharing on a root-covering remap, normalize detached and equals --remap-path-prefix forms, preserve the destination prefix, and keep malformed/external values conservative
• add Rust parser/context/request-fingerprint tests plus ignored rustc integration tests for file!() auto-remap hits, no-remap misses, different destination-prefix misses, and existing sibling dependency invalidation
• update README Rust worktree-sharing docs for the now-supported auto directive

Validation

• cargo test -p zccache-depgraph remap_path_prefix
• cargo test -p zccache-depgraph rustc_context_key_with_root
• cargo test -p zccache-daemon rust_remap
• cargo test -p zccache-daemon effective_compile_args_auto_
• cargo test -p zccache-daemon --test rustc_cache_test test_rustc_path_remap_auto_file_macro_hits_across_sibling_git_roots -- --ignored --nocapture
• cargo test -p zccache-daemon --test rustc_cache_test test_rustc_path_remap_conservative_cross_root_misses -- --ignored --nocapture
• cargo test -p zccache-daemon --test rustc_cache_test test_rustc_sibling_git_worktree_equivalent_cache_sharing -- --ignored --nocapture
• cargo test -p zccache-daemon --test rustc_adversarial_test rustc_remap_path_prefix_affects_cache_key -- --ignored --nocapture
• cargo test -p zccache-daemon --test stress_test path_remap_auto_compiles_file_macro_stably_across_git_roots -- --ignored --nocapture
• cargo test -p zccache-daemon --test link_cache_test test_link_path_remap_auto_hits_across_sibling_git_roots -- --ignored --nocapture
• cargo test -p zccache-daemon --test perf_bench_test perf_rustc_zccache_vs_sccache -- --ignored --nocapture

Benchmark

Rust benchmark on this machine (C:\ProgramData\chocolatey\bin\rustc.exe), 50 .rs files, 5 warm trials:

Scenario	Bare rustc	sccache	zccache	vs sccache	vs bare rustc
Build, Cold	6.727s	8.303s	7.710s	1.1x faster	1.1x slower
Build, Warm	6.344s	7.448s	0.135s	55x faster	47x faster
Check, Cold	4.345s	5.796s	5.147s	1.1x faster	1.2x slower
Check, Warm	4.025s	4.983s	0.103s	49x faster	39x faster

Build mode is --emit=dep-info,metadata,link; check mode is --emit=dep-info,metadata.

[codecov]

Codecov Report

❌ Patch coverage is 77.16263% with 66 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zccache-daemon/src/server.rs	74.21%	66 Missing ⚠️

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