feature - complete std.compression RFC (#339, #548)

[dannymeijer]

Summary

This PR completes RFC 061 std.compression and keeps the implementation dogfooded through source-authored Incan stdlib modules. It now covers one-shot and stream compression/decompression for gzip, zlib, deflate, zstd, bz2, lzma, framed Snappy, raw Snappy interop, explicit autodetection, generated-project compile coverage for issue #548, public reference and how-to docs, release notes, and moves RFC 061 to closed/implemented for 0.3.0.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/RFCs/*)

Area(s)

Select the primary areas touched (used for review routing; labels are managed separately):

• Incan Language (syntax/semantics)
• Compiler (frontend/backend/codegen)
• Tooling (CLI/formatter/test runner)
• Editor integration (LSP/VS Code extension)
• Runtime / Core crates (stdlib/core/derive)
• Documentation

Key details

• User-facing behavior: std.compression now exposes the RFC baseline codec APIs, including compress, decompress, compress_stream, decompress_stream, decompress_auto, decompress_auto_stream, shared Codec / CompressionError vocabulary, framed Snappy defaults, and raw Snappy under std.compression.snappy.raw.
• Internals: the compression modules stay source-authored in Incan rather than adding new Rust extern type implementations; the stream paths use BytesIO | File branches and shared stdlib helpers, and the generated-project test now builds/runs the nested Cargo output that originally failed in bug - Generated stdlib modules fail Rust build for source-defined sibling imports #548.
• Risks: the stream implementation intentionally uses concrete BytesIO/File branches until the compiler can lower a generic reader/writer helper without imposing backend-incompatible bounds. Codec behavior is covered by the fixture, but deeper cross-tool compatibility remains normal stdlib surface risk.

Testing / verification

• make test / cargo test
• make examples (if relevant)
• incan fmt --check . (if relevant)
• Manual verification described below

Manual verification notes:

• git fetch origin main
• git merge origin/main
• cargo build --bin incan
• incan build tests/fixtures/valid/std_compression_surface.incn
• target/incan/.cargo-target/release/std_compression_surface
• cargo test --test codegen_snapshot_tests test_std_compression_modules_compile_codegen -- --nocapture
• cargo test --test integration_tests test_std_compression_surface_runs_generated_project -- --nocapture
• make docs-build
• make fmt
• make pre-commit passed, including full nextest suite, clippy, cargo-deny, and smoke-test-fast.
• Source-quality and architecture review/fix loops completed with no remaining findings.

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): workspaces/docs-site/docs/language/how-to/compression.md, workspaces/docs-site/docs/language/reference/stdlib/compression.md, workspaces/docs-site/docs/RFCs/closed/implemented/061_std_compression.md, workspaces/docs-site/docs/release_notes/0_3.md

RFC lifecycle state: RFC 061 is implemented, moved to workspaces/docs-site/docs/RFCs/closed/implemented/061_std_compression.md, and marked shipped in 0.3.0.

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions

Closes #339
Fixes #548