feat: cache steps and export post-build metadata

[wolfv]

Stack

3 of 4 in the experimental build-steps series.

• Previous: feat: add packaged reusable build-step providers #2764 — packaged providers
• This PR: feat: cache steps and export post-build metadata #2765 — caching and post-build metadata
• Top: feat: generate recipes with pre-solve metadata backends #2767 — pre-solve metadata backends

Summary

Add persistent named-step caching and a line-oriented post-build metadata protocol.

Step caching

• expose RATTLER_BUILD_STEP_CACHE
• support hash- and mtime/size-based input/output declarations
• store executor-owned .state.json identities
• invalidate on script, cwd, effective environment, secrets, dependency solution, declarations, and missing outputs
• replay generated metadata on cache hits and remove stale outputs deterministically
• frame hashed paths/content to avoid ambiguous fingerprints

Post-build metadata

• expose per-section OUTPUT_FILE
• accept simple cat-friendly dotted paths with JSON values for arrays and objects
• update run requirements, run constraints, every run-export category, about, licenses, file selection, prefix handling, dynamic-linking settings, and post-processing
• reject build/host mutations after their environments have already been solved
• synchronize emitted requirements into package metadata

Examples and documentation

• add a cross-platform cache example demonstrating hits and invalidation
• document the output protocol and mutable fields

Validation

• cargo check --all-targets
• cargo clippy --all-targets -- -D warnings
• focused cache and recipe-output unit tests
• 5 layer-specific build-step/cache/output E2E tests
• cargo fmt --all --check
• git diff --check

Full cross-platform end-to-end coverage additionally runs on the top PR, #2767.