feature - implement std.regex RFC 059 (#294)

[dannymeijer]

Summary

This PR implements RFC 059 by adding std.regex as the safe-default regular expression surface for compiled patterns, matches, captures, splitting, and replacement. The public API and behavior live in Incan stdlib source: regex.incn imports regex::Regex / RegexBuilder directly through Rust interop, materializes borrowed engine matches/captures in Incan code, and keeps Rust limited to the actual external regex crate binding.

The PR also adds RFC 100 for the next std.re slice so Pythonic/backtracking-capable regex can share infrastructure without weakening the std.regex safety contract.

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: from std.regex import Regex, RegexError now provides compiled regex values with constructor flags, is_match, find, find_iter, captures, captures_iter, full_match, split, splitn, literal/capture-aware replacements, callable replacements, match spans, indexed/named captures, and explicit None for unmatched optional groups.
• Compiler interop: Direct Regex(...) construction lowers through the generic __incan_new hook rather than regex-specific hardcoding. rusttype aliases now resolve method calls through their underlying Rust canonical path, and metadata-backed Rust calls record borrowed scalar return coercions (&str/&[u8] -> owned Incan values) generically.
• Stdlib dogfooding: std.regex now dogfoods Incan source for compile flags, match/capture wrapper types, iterators, full-match logic, split/splitn, replacement interpolation, literal replacement, callable replacement, and borrowed engine materialization. The previous incan_stdlib::regex Rust snapshot module has been removed.
• Tooling hygiene: CLI structured logs now write to stderr so rust-inspect warnings cannot contaminate generated program stdout.

Testing / verification

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

Manual verification notes:

• make pre-commit-full-gate passed after merging origin/main: formatting, rustdoc gate, 2349 nextest tests, clippy, and cargo-deny all passed.
• make smoke-test-fast passed: release build, rust-inspect focused tests, assertion canary, web/nested project builds, examples runner, and benchmark build smoke all passed.
• cargo test -p incan std_regex passed.
• incan run tests/fixtures/valid/std_regex_surface.incn passed.
• Added end-to-end regex fixture coverage for matching, captures, iterators, split/splitn, replacement strings, literal replacements, callable replacement, bounded replacement, and unsupported safe-engine pattern diagnostics.
• Added compiler regression coverage for rusttype aliases resolving underlying Rust methods and borrowed Rust method returns being owned as Incan str.
• Added a layering guard proving std.regex uses direct rust::regex interop and preventing a Rust runtime-helper module from returning.

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/reference/stdlib/regex.md, workspaces/docs-site/docs/release_notes/0_3.md, workspaces/docs-site/docs/RFCs/closed/implemented/059_std_regex.md, workspaces/docs-site/docs/RFCs/100_std_re_pythonic_regex.md

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 #294