Reuse shared paths and source offsets

[stef-k]

Summary

• establish one immutable invocation context with canonical root, one configuration document, and stable selected-file identities for all current and future guards
• eliminate repeated path canonicalization/reporting reconstruction and source-prefix scans while preserving deterministic public behavior
• add a fail-closed, non-CI Wayfarer benchmark harness plus focused path, immutability, mapping, and O(1) identity regressions
• update usage documentation and changelog

Validation

• fresh installed-wheel discovery: 318/318 passed
• repository dogfood: expected REVIEW, --ci exit 0
• independent review: READY FOR PR at e59393fdc1b8966caf0277241a34f0dd917498d2
• git diff --check: passed
• exact Wayfarer snapshot/config benchmark: all subprocesses exit 0, outputs byte-identical, target unchanged

Variant	0.3.0 median	Candidate median	Improvement
LOC-only	36.299s	3.108s	91.44%
Syntax-only	45.869s	10.210s	77.74%
Normal six-guard	56.357s	10.873s	80.71%

Profile totals fell from 88,195,622 calls / 82.002s to 36,334,800 / 18.528s. Repeated relative_or_absolute_path, legacy reporting_path, _byte_at_point, and bytes.index hotspots were eliminated. Final reporting_path_for lookup is O(1) and measured 0.034s cumulative across 46,680 calls.

Closes #122