Refine AC-0001 spec for implementation readiness

[danieldilly]

Summary

• Refactored FS-0007 v0 multiplayer spec from 749→287 lines (62% reduction)
• Added ADR-0006 (Input Tick Targeting) and ADR-0007 (StateDigest Algorithm)
• Revised ADR-0003 to remove dt_seconds footgun, added tick-driven API
• Tightened Constitution: DM-0026 (InputSeq ties), INV-0003 (player identity), INV-0007 (ordering)
• Added PlayerId type alias and mechanical verification (entity ordering, tick postconditions, replay integrity)

Trace Block

Issue: #7
Spec: docs/specs/FS-0007-v0-multiplayer-slice.md
Constitution: INV-0001, INV-0002, INV-0003, INV-0004, INV-0005, INV-0006, INV-0007, DM-0001, DM-0006, DM-0008, DM-0011, DM-0014, DM-0016, DM-0017, DM-0018, DM-0019, DM-0020, DM-0023, DM-0024, DM-0025, DM-0026, DM-0027, AC-0001, KC-0001, KC-0002
ADRs: ADR-0001, ADR-0002, ADR-0003, ADR-0004, ADR-0005, ADR-0006, ADR-0007

Files Changed

Modified (12):

• docs/adr/0003-fixed-timestep-simulation.md — Complete revision: removed dt_seconds, added tick-driven API
• docs/adr/0005-v0-networking-architecture.md — Status: Proposed→Accepted
• docs/constitution/acceptance-kill.md — Fixed AC-0001.5 clarification wording
• docs/constitution/domain-model.md — Added DM-0025/26/27, tightened DM-0026 (InputSeq u64, tie-breaking)
• docs/constitution/invariants.md — Updated INV-0003 (player identity), INV-0007 (ordering)
• docs/constitution/id-catalog.json — Regenerated for new IDs
• docs/constitution/id-index-by-tag.md — Regenerated
• docs/constitution/id-index.md — Regenerated
• docs/networking/v0-parameters.md — Added max_future_ticks=120, input_lead_ticks=1
• docs/specs/_TEMPLATE.md — Removed redundant Issue section
• scripts/spec_lint.py — Removed "Issue" from REQUIRED_SECTIONS

New (3):

• docs/adr/0006-input-tick-targeting.md — TargetTickFloor semantics, InputSeq, input pipeline
• docs/adr/0007-state-digest-algorithm-canonical-serialization.md — FNV-1a 64-bit, canonicalization rules
• docs/specs/FS-0007-v0-multiplayer-slice.md — Implementation-ready v0 multiplayer spec (287 lines)

Deleted (1):

• docs/specs/FS-TBD-v0-multiplayer-slice.md — Replaced by FS-0007

Determinism / Simulation Impact

• This PR changes simulation-plane logic

Changes:

• Added StepInput (DM-0027) type for simulation input consumption (PlayerId + payload only)
• Revised tick-driven API in ADR-0003: advance(tick, step_inputs) with precondition tick == world.tick()
• Added mechanical verification: entity ordering by EntityId (INV-0007), tick postconditions
• Defined StateDigest algorithm (ADR-0007): FNV-1a 64-bit with canonicalization (-0.0→+0.0, NaN→quiet NaN)

Verification:

• Determinism preserved: All changes maintain INV-0001 (deterministic simulation), INV-0002 (fixed timestep)
• Replay scope unchanged: v0 = same binary + same platform (ADR-0005)
• No simulation code implemented yet; this PR is governance + spec only
• Tier-0 gate plan in spec provides mechanical test checklist for implementation

Verification

• just ci — All checks pass (fmt, lint, test, ids, spec-lint)
• just ids — Constitution IDs validated
• just spec-lint — FS-0007 spec structure validated
• Constitution docs changed: just ids-gen run, outputs committed (id-catalog.json, id-index.md, id-index-by-tag.md)

Risk / Notes

Key architectural decisions formalized:

• TargetTickFloor (DM-0025): Server-emitted tick floor for client input targeting; eliminates past-tick ambiguity
• InputSeq (DM-0026): Per-session monotonic u64 for deterministic input selection; greatest wins, ties handled
• StepInput (DM-0027): Clean simulation-plane input type; no protocol metadata leakage

Constitution tightening:

• INV-0003 now explicitly states PlayerId binding is Server Edge responsibility (trust boundary)
• INV-0007 now mandates entity iteration ordering (deterministic, not just stable)
• DM-0026 (InputSeq) now u64 with explicit tie-breaking (was ambiguous)

Spec refactoring rationale:

• Removed ~460 lines of redundant prose (Component Responsibilities, Boundary Contracts, verbose rationale)
• Moved StateDigest algorithm to ADR-0007 (single source of truth)
• Lean spec focuses on interfaces, protocol, gate plan — sufficient for implementation

Follow-ups (not blocking):

• Implementation PR will add Tier-0 tests per gate plan
• Spec status will change Draft→Approved→Implemented after review and implementation

Checklist

• Trace block is complete and accurate
• This PR stays within Constitution invariants (no silent weakening; INV-0003/0007 tightened)
• No new dependencies added
• Spec exists for non-trivial changes (FS-0007)
• Gates added per spec's Gate Plan (15 Tier-0 items; implementation PR will add tests)