TUI: extend LayoutBudget with horizontal geometry

[TZZheng]

Summary

• extend the root-owned LayoutBudget from vertical-only sizing to one two-axis geometry contract
• add terminal/content/rail/minimum-chat widths and rail visibility, while keeping the production rail request at zero so current UI pixels do not change
• route both raw and synthesized resize paths through the same ChildWindowSize() content rectangle
• exercise real nonzero-rail threshold arithmetic in the pure resolver without shipping a visible rail
• document the new ownership in tui/internal/tui/ANATOMY.md

Part of #645 (the third independent root PR in the issue's implementation DAG). This PR does not add the rail widget, Agent selection, input rerouting, or thread/cache behavior.

Behavior and scope

resolveHorizontalLayout owns the one allowed horizontal partition. A rail is visible only for an allowed view and only when its requested width leaves at least the minimum chat width; hidden rails consume zero columns. App.layoutBudget() formally requests a zero-width rail in this PR, so Mail and non-Mail views retain the full terminal width.

The named minimumChatWidth default is 60 columns. It is currently inert because the requested rail width is zero; the threshold and exact partition behavior are covered directly for later visible-rail work.

Test-first evidence

Before implementation, the focused contract suite was written and run against base d0167c35d696654899678e5a5498df04ebab6492:

go test ./internal/tui -run 'TestLayoutBudgetOwnsHorizontalGeometryWithoutChangingCurrentWidth|TestLayoutBudgetHorizontalTinyAndThresholdWidthsAreSafe|TestRawAndSyntheticResizeShareMailHorizontalGeometry|TestNonMailViewKeepsFullTerminalWidth' -count=1

It exited 1 after compiling and executing. The intended failures were that the base LayoutBudget had no TerminalWidth, MinChatWidth, or ContentWidth fields; this was not a syntax, import, dependency, environment, or unrelated flaky-test failure. The contemporaneous RED record has SHA-256 90ead718729e5924e89a74516054f8a4df6fb9e4f882694b55b4b5bf77f25384.

The initial RED tests used reflection so they could compile against missing fields. After implementation, the committed tests were tightened to direct typed field access.

Validation

Exact candidate: 3c03fe0494870e1d8247d1c7b58b714d4996b606

# focused layout, resize, chrome, Mail, and non-Mail contract
go test ./internal/tui -run 'TestLayoutBudget|TestUpdateForwardsReducedHeight|TestUpdateNoChromeForwardsFullHeight|TestSendSize|TestRawAndSyntheticResizeShareMailHorizontalGeometry|TestNonMailViewKeepsFullTerminalWidth|TestViewComposesBannerOnce|TestVisitingDoesNotReserveRootChrome' -count=1

# full TUI module suite
go test ./...

git diff --check

All passed. The repository-shipped anatomy citation checker also passed with no missing or out-of-range targets, and the changed production-file citations were semantically inspected.

Independent read-only review of this exact HEAD:

• Claude: PASS, no blockers; independently ran build, full tests, vet, formatting, and diff checks
• Codex (gpt-5.6-sol, high reasoning): PASS, no blockers; independently ran focused/full tests, vet, formatting, citation, and cleanliness checks

Both reviewers explicitly judged the inactive 60-column minimum acceptable for this foundation PR.

Cleanup and bridge ledger

• child views do not subtract rail width themselves
• raw and synthesized resize paths share the existing root dispatcher and one computed content rectangle
• package anatomy is updated in the same commit
• bridge ledger: empty; the zero rail is the issue-specified behavior-preserving foundation, not a feature flag

Rollback

Reverting this commit restores the former height-only budget and single-width forwarding. No persistence or data migration is involved.