fix: send SIGWINCH to PTY when terminal element resizes

[Wirasm]

Summary

• Store PTY master handle in Terminal struct (via Arc<Mutex<>>) instead of moving it into the reader thread
• Add ResizeHandle struct that bundles resize refs (term, pty_master, current_size) with a resize_if_changed() method
• Call resize_if_changed() from TerminalElement::prepaint() when computed dimensions differ from stored size
• Sends SIGWINCH to child process and reflows terminal grid on resize

Programs like vim, htop, and shell prompts now respond correctly to window resize. $COLUMNS/$LINES reflect actual terminal dimensions.

Closes #310

Test plan

• cargo fmt --check passes
• cargo clippy --all -- -D warnings passes (0 warnings)
• cargo test --all passes (494 passed)
• cargo build --all succeeds
• Manual: run kild-ui, verify echo $COLUMNS matches window width
• Manual: resize window, verify echo $COLUMNS updates
• Manual: open vim, resize window, verify vim redraws correctly

[Wirasm]

PR Review Summary

Critical Issues (1 found)

Agent	Issue	Location
code-reviewer, silent-failure-hunter	.expect() on current_size lock panics on poison — crashes entire UI	state.rs:409-412

Important Issues (2 found)

Agent	Issue	Location
code-reviewer, silent-failure-hunter	pty_master.lock() failure is completely silent — no logging	state.rs:420
silent-failure-hunter, type-design-analyzer	TerminalError::PtyResize variant defined but never used	errors.rs:30-31

Suggestions (3 found)

Agent	Suggestion	Location
type-design-analyzer	Return Result<(), TerminalError> from resize_if_changed() to surface failures and use the PtyResize variant	state.rs:406
comment-analyzer	SIGWINCH comment should say "updates kernel winsize, triggering SIGWINCH" (not "sends SIGWINCH")	state.rs:419
comment-analyzer	Lock ordering comment should document that each lock is scoped and released before acquiring the next	state.rs:405

Strengths

• Clean architectural separation via ResizeHandle — keeps resize logic out of both Terminal and TerminalElement
• Efficient idempotency check prevents unnecessary PTY syscalls
• Well-documented lock ordering prevents deadlocks
• Correct prepaint ordering (resize before snapshot)
• Good Arc ownership model for PTY master keepalive

Type Design (7.5/10)

Dimension	Score
Encapsulation	8/10
Invariant Expression	6/10
Invariant Usefulness	9/10
Invariant Enforcement	7/10

Verdict: NEEDS FIXES

Two error handling issues violate the project's "No Silent Failures" principle:

1. Must fix: Replace .expect() with match/log/return to prevent UI crash on lock poison
2. Should fix: Log pty_master lock failures explicitly — currently completely silent
3. Should fix: Either use PtyResize error variant or remove it

Core implementation is solid. Fixes are straightforward and don't require architectural changes.

Recommended Actions

1. Replace .expect("current_size lock poisoned") with match + tracing::error! + early return
2. Add tracing::error! logging when pty_master.lock() fails (currently silent if let Ok)
3. Either make resize_if_changed() return Result<(), TerminalError> (using PtyResize variant) or remove the unused variant
4. Minor comment accuracy fix for SIGWINCH mechanism description