docs: Phase 3 exit decision; deferrals move to Phase 4

[dvzrd]

Operator decision (2026-10-06): Phase 3 is closed.

• Adds the Exit decision section to phase-3-alpha.md: each criterion, its evidence and result.
• Phase 3 done; roadmap table, README Status line, development guide, Phase 4 entry criteria and the dogfood log updated. Phase 4 stays proposed.
• Deferred to Phase 4 as P4-12 to P4-15: P3-01, P3-33, P3-35, P3-36. P3-38 (V23) is dropped as not reproducible (chore: P3-38 V23 investigated and dropped; guard the unread-master exit #188).
• P3-39 (first crates.io release) now waits only on the second drift check and the Release workflow dry run.

Verified: link-check and rumdl pass.

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