Enforce canonical multiline XML documentation

[serene1491]

Summary

Make multiline XML documentation the only canonical source form for every non-empty element. The formatter now expands valid inline elements safely, preserves malformed input, code whitespace, indentation, and line endings, and inserts canonical separators between top-level contract elements.

This PR also migrates all repository documentation and fixtures, prevents inline regressions, restores unique ADR identities after the recent merge, and replaces potentially truncating native range conversions with checked conversions.

Architecture traceability

• Authorizing architecture section or ADR: ADR 0057 - Multiline XML Documentation Format
• New or changed public contract: Non-empty XML documentation elements use separate opening, content, and closing --- lines. Sibling top-level elements use one empty --- separator. Self-closing syntax remains available only for genuinely empty elements.
• Architecture documents, examples, or terminology updated: XML documentation architecture, syntax and nomenclature, closed decisions, architecture index, compiler fixtures, standard-library source, and ADR cross-references. Duplicate implementation ADRs were renumbered to 0058-0060 with traceability notes.

Verification

• Tests were added or updated before implementation where behavior changed.
• Positive behavior is covered.
• Negative/rejection boundaries are covered.
• Convention, consistency, and regression coverage is present where relevant.
• Cross-backend or differential coverage is present where relevant.
• cargo fmt --all -- --check
• cargo check --workspace --all-targets
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings

If a check was not run, explain why:

The full Clippy command was run. It remains unsuccessful because the current pop-types baseline reports 59 pre-existing warnings promoted to errors, primarily complexity, wildcard-import, documentation, and repeated-match-arm lints from the recent generic/iteration work. Clippy passes with -D warnings --no-deps for the formatter, documentation, architecture-test, and native-runtime components changed here. Three range conversion lints and one documentation lint discovered during this review were fixed.

Additional successful validation:

• formatter, documentation, architecture, native ABI, LLVM, and MIR interpreter tests;
• strict repository scan reporting zero inline XML documentation bodies across Pop and the companion skills repository;
• Markdown link validation;
• ADR identity/status validation;
• stale ADR-path validation;
• skill quick_validate.py validation;
• zero trailing whitespace in Markdown and Pop source.

Review notes

• No dynamic typing, runtime string lookup, broad reflection, or universal-table behavior was introduced.
• HIR/MIR remain backend-neutral.
• No generated artifacts, dependency caches, credentials, or editor files are included.
• This is ready for technical review.

The formatter library implements the XML documentation rule. The complete user-facing pop format command remains roadmap work and is not claimed by this PR.

[kleeedolinux]

Approved