docs(post-A.5): capture holtburger network-stack study + Phase M.0 quick-wins

Holtburger reference fast-forwarded from 88b19bd to 629695a (+237 commits).
Four parallel research agents produced a parity-first-pass between
holtburger's network stack and acdream's src/AcDream.Core.Net/.

Why captured now: study surfaced six small, high-confidence "Tier 1" fixes
that can ship before the bigger M.1-M.8 layer extraction. Most likely fix
for the longstanding "remote retail observer sees us not perfect" bug
(MoveToState wire-format mismatches). Two transport gaps (no EchoResponse
reply, eager port-switch) match recent holtburger fixes (403bc98, 99974cc).
One latent bug worth a 5-min check (ISAAC search-mode for out-of-order
ENCRYPTED_CHECKSUM packets).

Captured as Phase M.0 in the roadmap so the work survives the session and
can be picked up later. Existing M.1-M.8 lift unchanged; M.1 marked as
partially started since the research note is the parity-map deliverable
in draft form.

Files:
- docs/research/2026-05-10-holtburger-network-stack-study.md (new) — full
  study with ranked port candidates, recent commits worth knowing, and
  acdream-vs-holtburger file map.
- docs/plans/2026-04-11-roadmap.md — Phase M Plan-of-record updated with
  2026-05-10 pointer; M.0 sub-lane added before M.1; M.1 status note.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/plans/2026-04-11-roadmap.md

@@ -438,13 +438,51 @@ implementation starts. Treat holtburger as the client-behavior oracle for this
 phase; cross-check wire details against named retail, ACE, Chorizite, and AC2D
 before porting.
 
+**2026-05-10 update:** holtburger pulled to `629695a` (+237 commits since
+last audit). First parity-pass written to
+[`docs/research/2026-05-10-holtburger-network-stack-study.md`](../research/2026-05-10-holtburger-network-stack-study.md)
+— that doc is the M.1 deliverable in draft form. Study identified six
+high-ROI "Tier 1" fixes that are individually small and can ship as a
+focused pre-pass before the bigger M.1-M.8 lift; tracked as **M.0** below.
+Most relevant recent holtburger commits to consult: `99974cc` (session
+crate split + retransmit core), `403bc98` (port-switch race), `336cbad`
+(turning + locomotion fix), `797aece` (disconnect carries client_id).
+
 **Sub-lanes:**
+- **M.0 — Tier 1 quick-win polish pre-pass.** Six small, high-confidence
+  fixes that don't require the full M.1-M.8 layer extraction and can ship
+  as one focused PR (~1 day). Sourced from
+  [`docs/research/2026-05-10-holtburger-network-stack-study.md`](../research/2026-05-10-holtburger-network-stack-study.md)
+  §1 Tier 1. May ship independently of M.1-M.8.
+  1. **MoveToState wire-format audit** (study §1.1.a-e). Side-by-side
+     compare `Messages/MoveToState.cs` against holtburger
+     `client/movement/common.rs:122-186`. Pin: `current_hold_key` always
+     set, empty `commands[]` on held WASD, `turn_speed` always with
+     TURN_COMMAND, gait-aware dedup, no `turning` when locomotion ≠ 0.
+     Likely candidate for the longstanding "remote retail observer sees
+     us not perfect" bug.
+  2. **LoginComplete on every PlayerTeleport** (study §1.2). Currently
+     only sent on first PlayerCreate.
+  3. **EchoRequest → EchoResponse reply** (study §1.3). We parse and
+     ignore; ACE pings periodically — likely contributor to long-session
+     timeouts.
+  4. **Port-switch race fix** (study §1.4, holtburger commit `403bc98`).
+     Track pending vs confirmed `_connectEndpoint`.
+  5. **Disconnect packet carries client_id** (study §4, holtburger commit
+     `797aece`). Currently `id = 0`.
+  6. **Verify `IsaacRandom` has search-and-stash mode for out-of-order
+     ENCRYPTED_CHECKSUM packets** (study §1.7, holtburger
+     `crypto.rs:73-93`). 5-minute check; ~20 LOC port if missing —
+     latent bug under any UDP reorder event.
 - **M.1 — Audit & parity map.** Produce a source-by-source comparison of
   acdream `AcDream.Core.Net` and holtburger `holtburger-session`,
   `holtburger-protocol`, and `holtburger-core` networking code. Inventory each
   packet flag, optional header, session transition, control packet, fragment
   path, game message, and game action. Mark each as `parity`, `partial`,
-  `missing`, or `intentional divergence`.
+  `missing`, or `intentional divergence`. **Status (2026-05-10): first pass
+  done at [`docs/research/2026-05-10-holtburger-network-stack-study.md`](../research/2026-05-10-holtburger-network-stack-study.md);
+  the formal parity table can extend that doc rather than start from
+  scratch.**
 - **M.2 — Layer extraction.** Split the low-level stack under `WorldSession`
   into testable components: `INetTransport`, `PacketCodec`,
   `ReliablePacketSession`, `FragmentSession`, `GameMessageSession`, and the