[DOI] proxy URN-colon carrier + future allocation triggers (deferred v1 scope) #174

Follow-up to #173 (DOI capability, MILESTONE row 11). Three items were deliberately deferred during v1; only the first is actionable now.

1. Proxy URN-colon carrier (actionable)

The DOI proxy understands colon-for-initial-slash substitution (Handbook §6.3.1 NOTE): https://doi.org/urn:doi:10.123:456 resolves 10.123/456, with multi-slash suffixes hex-encoded (10.123/456ABC/zyz → …:456ABC%2Fzyz). v1 recognizes bare urn:doi: but never maps the colon, so these read MISSING (pinned by test_urn_colon_form_missing in tests/capabilities/doi/test_grammar.py).

Acceptance: new carrier group (colon→slash mapping + %2F rule) resolving https://doi.org/urn:doi:10.123:456 → SUCCESS 10.123/456 (prefix must still satisfy the 4–9 registrant bound, so 10.123:456 stays MISSING); distinct grammar semantics id per the affinity rule (research §4.5); natural fit for an extra_grammars community extension rather than the shipped grammar.

Refs: research §13 row 11 (docs/development/research/2026-09-18-doi-canonicalization.md), plan Task 3 (docs/development/plans/2026-09-18-doi-capability.md), Handbook local conversion docs/development/research/DOIHandbook_2025.pdf.md:2298-2307.

2. Future triggers (not actionable — revisit only if the Foundation acts)

• Non-10. directory indicators (Handbook §4.3.2 reserves them): v1's 10. gate reads them MISSING by design.
• ≥10-digit registrants (Handbook: no defined length limit): v1's {4,9} bound encodes current allocation practice.
• Either allocation change requires a recognition_revision bump and a bound/gate revisit (research §13 row 4). No action until observed in the wild.