fix(engine,web): F1 bound every terminal replay (WI-125)

[thedancingdeveloper]

Second PR of the terminal-attach-budget initiative (WI-121). Stacked on #667 (H1) — its base is feat/terminal-attach-h1-harness, so this diff shows only F1's changes; both land on feat/terminal-attach-budget, nothing to main until H3 is green. Merge #667 first (or merge this into the feature branch after it).

The bug (WI-125)

When a warm reattach's resume cursor had aged out of the ring (a parked session produced more than the 4 MiB ring while away, or the cache was stale), snapshot_for_attach returned the whole untrimmed ring with reset:true — up to 4 MiB parsed into an xterm that keeps 5000 lines, 4× worse than a cold attach (capped at 1 MiB), and painted above a screen the client already had stale. An oversized in-window delta had the same problem.

The fix

Engine (pty.rs) — snapshot_tail_bytes is now the replay budget on every path:

• warm, cursor retained, delta ≤ budget → byte-exact delta, reset:false (an ordinary switch-away/switch-back still appends);
• warm, cursor aged out or delta > budget → ground-state-aligned tail ≤ budget, reset:true;
• cold → bounded tail (unchanged);
• in-band resync (tail_bytes=None) → unchanged.

Web (api.ts) — openAttach now always sends snapshot_tail_bytes alongside resume_from. Terminal.tsx already resets + clears cache on any reset:true; the snapshot-start position arithmetic is extracted to snapshotStartPosition() and unit-tested, so the re-anchor after a reset-following-a-resume (discard stale cursor → re-anchor to scrollback_pos - scrollback_bytes → resume gap-free) is pinned.

Tests

• H1 large_stale_warm_reattach_* and four_mib_ring_stale_reattach_* "before" flood assertions flipped to bounded (len ≤ budget, reset:true, ground-state aligned).
• warm_attach_is_not_narrowed_by_a_tail_hint inverted → warm_attach_over_budget_is_a_bounded_reset; new warm_attach_within_budget_stays_a_byte_exact_delta keeps the reset:false byte-exact case.
• web: new snapshotStartPosition.test.ts; attachAuthFrame.test.ts updated for the always-sent budget.

Verification

• cargo test -p vogt-engine-server --test integration — 104 passed; cargo fmt --check + clippy --tests clean.
• pnpm typecheck clean; pnpm vitest — 906 passed.
• scripts/check_docs.py clean. docs/ENGINE.md §5 documents the budget contract (reset:true may follow a resume_from).

🤖 Generated with Claude Code

https://claude.ai/code/session_01UtEFLJAhLiq4NfZCN2vuBb

[thedancingdeveloper]

Cleanup: superseded by the single promotion PR #676. All of this work is already integrated into feat/terminal-attach-budget and is reviewable there per-commit (each commit is one initiative item with a full message). Closing to keep the initiative to one PR; the commit is preserved.