fix: require attach before resize

[schickling-assistant]

Why

A fresh command socket can currently send RESIZE before ATTACH. That admits
it as a writable geometry participant and changes the shared grid even though
the socket has not requested a screen baseline.

What

• Require a complete writable ATTACH before accepting RESIZE.
• Add a causal protocol regression that preserves pre-ATTACH DATA and
  STATUS while proving the command socket cannot affect geometry or attached
  client counts.
• Specify command, readonly, and writable-attached capabilities consistently in
  the VRS documents.

Rationale

The fix reuses attachSeq > 0, which is already the server's predicate for
stats, geometry negotiation, and geometry broadcasts. It preserves intentional
command input without introducing a new protocol frame or broad role-state
refactor.

Verification

• Focused regression: passed.
• TypeScript build: passed.
• Typecheck: passed.
• VRS verification: passed, 2 documents / 11 requirement IDs.
• Diff check: passed.

Reproduction:
https://github.com/schickling-repros/2026-08-pty-resize-before-attach/tree/cef285ea85de3b750aa6dc341a560a58f77b07fd

Closes #153

Posted on behalf of @schickling

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[myobie]

I reviewed exact draft head 9e1eafc. I like this direction. The implementation is narrow: a fresh command socket can still send input and request status, but RESIZE is ignored until a complete ATTACH has established geometry membership. The regression also proves that the ignored resize does not change terminal geometry, emit GEOMETRY, start a screen baseline, or add the command socket to public attached-client stats. Hosted build and Vitest checks are green.

One non-blocking design note: !client.readonly && client.attachSeq > 0 represents the role through two existing fields. That is correct for the current three roles. If the protocol adds another role later, an explicit role value could make the state machine easier to audit. I do not see that as a reason to expand this draft now.

[schickling-assistant]

Thank you, Nathan, for the careful push to clarify the actual client contract before changing resize behavior. The resulting boundary is much simpler: command sockets keep DATA and STATUS, while geometry authority begins only after writable ATTACH.

Posted on behalf of @schickling

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