attach: stop nudging a child that is already the right size; remove --no-resize

[schickling-assistant]

Now carries #110 as well, so this is the whole thing against main: remove geometry-neutral attach, and stop nudging a child that is already the right size.

The nudge

nudgeRedraw() (resize to cols - 1 and back) fired on every attach. Its purpose is to paper over serialize-replay artifacts, but when a client attaches at exactly the size the session already has, the child is by definition already drawn for that geometry — so the nudge buys nothing and just delivers SIGWINCH to an otherwise idle process every time someone connects.

It now fires only when the attaching client's geometry differs from the session's. The comparison is read before negotiateSize(): a smaller client shrinks the session to its own size, which would otherwise look like it had matched by the time the decision is made. That case still nudges, as it should.

--no-resize goes away

You asked whether fractal's embedded pane was read-only-with-crop or typed-into-while-cropped, and noted that read-only is already geometry-neutral so the feature wouldn't be strictly needed in the first case. The honest answer turned out to be that it isn't needed in either.

Fractal reads the session's geometry from pty stats and opens its attach client at that size, cropping into the pane — it never reports the pane's dimensions. A client whose geometry equals the session's is inert in negotiateSize() by construction. So the geometry half of the neutral role was already a no-op for its only consumer.

The 80×24 justification in #95 was a fractal defect — it launched pty attach over pipes — not a pty gap, and it has since been fixed on our side by giving the attach client a real PTY. What genuinely remained was the nudge, which is fixed above for every client rather than through an opt-in role.

What that leaves

The ATTACH frame goes back to a plain 4-byte payload. ATTACH_FLAG_GEOMETRY_NEUTRAL, decodeAttachFlags, the geometryNeutral client field, capabilities.geometryNeutralAttach, clients.geometryNeutral, requireGeometryNeutralAttach(), and tests/neutral-attach-cli.test.ts are all gone. cmdAttach / doAttach return to their pre-#96 signatures.

Net against main: about 230 lines removed, ~10 added.

I did briefly add the --force-resize flag you offered — then took it out again. You said we can always add it, and nothing needs it today; shipping an unused opt-in flag into your protocol would have repeated exactly the mistake this PR is undoing. If skipping the nudge does regress a child's layout, it's a small change to add then, and I'd rather that be driven by a real case.

Verification

npm run typecheck clean. protocol, integration, completions suites pass. Integration coverage: an ordinary attach at the session's current size produces no SIGWINCH in a quiescent child, and a client attaching at a different size still nudges.

Unrelated pre-existing failures in my environment (all also present on main): shells.test.ts and screenshot.test.ts need zsh / vim, which aren't installed here, and help.test.ts reports the completions command from #106 as undocumented.