[Spike] v2 UI: multi-client terminal resize policy in the v2 layer

[pseudoseed]

Spike 1 of the Codev v2 UI (#37). Experiment, not implementation. No production
code ships from this issue.

The problem

Two clients attached to the same terminal fight over dimensions. PtySession.resize()
(packages/codev/src/terminal/pty-session.ts:567) is a single global call, last-writer-wins.
Attach itself already fans out correctly (PtySession.clients is a Set, with tests), so
this is the one confirmed multi-client bug.

This blocks the entire two-device premise of v2: an iPad and a desktop viewing the same
builder is the product.

The hard constraint

Do not change PtySession's contract. That file is untouched by this fork and hot
upstream; editing it is a merge conflict paid at every sync, forever. See Part 0 of
codev/research/codev-v2-ui-frd.md.

Solve it in a v2-owned attach wrapper that negotiates per-viewer dimensions and calls
the existing resize() with a single agreed value.

Candidate policies to evaluate

1. Follow the focused client.
2. Ignore resize from hidden panes.
3. Per-viewer local cols with server-side reflow.

Pick one on evidence, not preference. Say what each costs and where each is wrong.

Deliverable

A written finding in codev/experiments/ covering:

• A reproduction of the bug as it exists today, with captured output rather than a description.
• A working prototype of the chosen policy in a v2-owned wrapper module, with git diff --stat
  proving pty-session.ts is untouched.
• What the policy does when the focused client disconnects, when both clients are hidden,
  and when a client reconnects after an iOS backgrounding drop.
• Whether FR-38 can be met additively, or whether it is a requirement to renegotiate.

Reference

• FRD: codev/research/codev-v2-ui-frd.md, Part 0 (the additive seam), FR-38, Appendix spike 1.
• Related: FR-39, idempotency keys on mutating RPCs, same backgrounding cause.

[pseudoseed]

Spike complete. PR #45 merged as 58fd5f5.

Finding: follow-the-focused-client, and it meets FR-38 additively. Hidden viewers never apply a size, a sole remaining viewer is promoted to focused, and a reconnecting client attaches visible but unfocused so its post-connect nudge cannot steal size from a peer. pty-session.ts is untouched, which was the constraint from Part 0.

Last-writer-wins was reproduced against current code rather than taken from the FRD: two attached clients, A at 80x24 then B at 40x12, with the live shell's own stty size reporting 12 40.

The two rejected policies were rejected on evidence. Ignore-hidden still fights when both clients are visible, which is the iPad-plus-desktop case the whole product rests on. Per-viewer reflow cannot be done additively, because one PTY has one size.

Notes and artifacts: codev/experiments/38-multi-client-resize/.

Two items carried into the v2 spec under #37, neither blocking:

1. FR-38 is additive only for v2-to-v2 attach. A v1 client still calls resize() through tower-websocket.ts and the REST path, and no v2 wrapper can stop it.
2. Focus is itself last-writer-wins when two viewers both send a focus frame. Treating input as implicit focus is the cheap fix.

Production module tracked under #37.