docs: fix unsafe WAL recovery advice and align docs with milestones - #408

Summary

This PR fixes doc problems found in a full docs and wiki review (see #383).

• Unsafe recovery advice (most important). docs/ERROR_REFERENCE.md WAL-001, WAL-002 and STG-011 told users one of two things:
  • they could delete the .wal file with no data loss ("committed facts are in the .graph file"), or
  • they could rebuild a database from the WAL alone.
    Both are wrong. Transactions committed since the last checkpoint exist only in the WAL, and the WAL holds nothing older than that checkpoint. The new text says to copy both files first, to checkpoint with the version that wrote the WAL, and what is lost if the WAL is deleted.
• Broken links. About 50 links in ERROR_REFERENCE.md pointed at ../../.wiki/..., a local clone path. They now use full wiki URLs, with anchors mapped to the real headings. One ../BENCHMARKS.md link was also wrong.
• ROADMAP and CLAUDE.md now match the live milestones:
  • v2.0.2 is the final planned v2.x release.
  • v3.0.0 is format v8 and data integrity.
  • v3.1.0 holds the features.
    CLAUDE.md now documents error.rs, magic_sets.rs, fault_inject.rs and src/browser/.
• README:
  • states the MSRV (1.89), and the edition badge no longer links to the Rust 1.82 notes;
  • shows the v2.x known issue (bug: same-transaction multi-valued facts read back as ONE value, differing between EAVT and AEVT; a batched retract can leave retracted values visible #371) with its workaround;
  • corrects the Maven coordinates to io.github.project-minigraf (checked on Maven Central, 2.0.1 is published there; nothing is under io.github.adityamukho);
  • lists Android on Maven Central;
  • points binding downloads to the binding repos (v2.x core releases carry no .wasm, C or Swift assets).
• SECURITY.md: 2.x is the supported line.

The PHILOSOPHY.md stability wording is left for #397, because it needs a policy decision. Wiki fixes are pushed separately to the wiki repo.

Type of change

• Documentation / tests only

Checklist

• cargo test passes (--lib error_reference and --lib registry, which parse ERROR_REFERENCE.md, were rerun; the full suite passed at 69da4b2 with 1,168 passed and 8 ignored)
• CHANGELOG.md updated under Unreleased
• I have read PHILOSOPHY.md and this change aligns with it

Notes for reviewer

Please check the new WAL-002 steps ("open with the version that wrote the WAL and call checkpoint()") against how you want downgrades handled.

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