feat(memory): procedural derived-stats compat helper + shadow-read drift

[stackbilt-admin]

Summary

Moves the aegis#564 Phase 2 compat helper down from the daemon into core so public `/procedures` + `/dashboard` readers actually exercise it. Daemon's shadowed duplicates were dead code (Hono first-match-wins, core mounts first), so drift samples only came from the router hot path at 10% sampling — effectively blinding the Phase 3 readiness gauge. Closes #30.

Added

• `episodic_memory.complexity_tier TEXT` column (aegis#563 prerequisite — the compat helper queries by it)
• `shadow_read_drift` table + index (Phase 2 drift log)
• `getEpisodeStatsByComplexity`, `getAllEpisodeStatsByComplexity`, `EpisodeStatsAggregate` in `kernel/memory/episodic.ts`
• `DriftLogOpts`, `getProcedureWithDerivedStats`, `getAllProceduresWithDerivedStats` in `kernel/memory/procedural.ts` (plus internal `logDriftSingle` / `logDriftBulk` helpers)

Swapped

• `dashboard.ts` → `getAllProceduresWithDerivedStats(db, { reader: 'dashboard' })`
• `routes/observability.ts /procedures` → `getAllProceduresWithDerivedStats(c.env.DB, { reader: 'observability' })`

Scope note (partial aegis#563)

This picks up a slice of aegis#563 as prerequisite — only the `complexity_tier` column on `episodic_memory`, which the compat helper queries. Other aegis#563 columns (`executor_config`, `court_card`) are not touched here; they can land independently if needed.

Generic-library delta from daemon

Dropped daemon's `DERIVED_CUTOFF = '2026-04-22'` date guard. That cutoff is daemon-specific (when its migration ran); core is generic, any other consumer migrates on a different day. Rows with `complexity_tier IS NOT NULL` are treated as derivable; consumers wanting stricter protection against retroactive backfills can wrap with their own time filter.

Test plan

• Core typecheck: clean
• Core tests: 1474 passed, 1 skipped (observability.test.ts mock updated to match new export)
• Daemon typecheck against this branch: clean
• Consumers need to rerun `schema.sql` — adds one column + one table
• Daemon follow-up PR: collapse `dashboard.ts` shadow + remove v2.1.6 `drift-probe` cron workaround

Refs aegis#564 Phase 2, aegis#563 (prereq), #35