docs(specs): FR5 family + ADR-0009 (loader error envelope + source-on-node)

Brainstormed re-scope of the original FR5 draft (which was JSON-only and
under-designed). The new structure:

ADR-0009 — cross-language contract:
- Uniform error envelope { code, message, source, ...optional T2 }.
- `source` on every metadata node (not a side-channel). Always populated;
  programmatic construction defaults to { format: "code" }. Mutates on
  real semantic change; duplicate-no-change emits WARN_DUPLICATE_DECLARATION.
- Canonical JSONPath form (dot for identifier keys, bracket for arrays/
  special-char keys, root $). Exact-string conformance enforcement.
- `semantic_diff` algorithm spec'd so all four ports converge.
- Six format discriminants: json, yaml, merged, resolved, database, code.
- Canonical JSON serialization explicitly omits `source` (loader-output,
  not metadata).

Per-error-class FRs:
- FR5a (plan-of-record): JSON-shape + envelope foundation. Coordinated
  TS/C#/Java/Python landing. Adds source field to each port's MetaData
  base, threads JSONPath through parse, rewrites ~12-15 conformance
  fixtures to the envelope shape.
- FR5b/5c/5d (sketches): YAML positions, multi-file merge attribution,
  reference resolution. Each owned later with its own brainstorm.
- FR5e (forward-looking): database-source errors, gated on FR-003.

Supersedes the original 2026-05-25-fr-actionable-loader-error-messages-
design.md (deleted; was a single under-scoped draft).

Roadmap updated to reflect the family.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dmealing