test(parity): R2 shared fixtures — screens.json (both exit modes) + shapes.json (json-shape)

[myobie]

Parity R2 — shared, language-neutral fixtures both suites assert

Round 2 of the parity drive: a single canonical set of behavioral fixtures both
the node suite and the Rust port (pty-rust) load and assert, so the two
implementations cannot silently drift. Node owns these files; pty-rust vendors
a byte-identical mirror. Rebased onto the post-#114 main.

screens.json — plain-screen bytes (exact) + both exit modes

Harness: tests/parity-fixtures.test.ts. Plain-screen output asserted EXACTLY
(trailing \n stripped, no full-trim). A per-fixture env overlay pins the
exit-time mode now that post-exit behavior is configurable (PTY_REAP_ON_EXIT,
#114):

• idle-prompt-plain — live prompt, trailing cursor cell kept ("READY> ").
• post-exit-final-screen — preserve (PTY_REAP_ON_EXIT=false): final
  viewport survives, idempotent peek, ls status=exited exitCode=7.
• post-exit-reaped — reap (default): session removes itself; peek fails +
  ls omits it.

shapes.json — machine-readable shapes (per-field policy)

Harness: tests/parity-shapes.test.ts. Runs a scenario via the real CLI, then
asserts field-by-field per policy ({exact} | {type} | {omitWhenUnset}),
not raw bytes:

• ls-json-shape — list --json entry shape, running vs exited (preserve so
  the exited entry stays listed). status enum running|exited|vanished.
  Documents the pid distinction: ls pid = daemon pid (number running / null
  exited), vs stats.process.pid = child pid.
• client-count-during-peek — after a transient peek, stats --json
clients.attached === 0.

Verification

All fixture values captured empirically against the real dist. All parity
suites green (screens 4, shapes 3, parity-node-reference 7); typecheck clean.
pty-rust has confirmed the screens loader + json-shape encoding; it mirrors both
files. README.md documents the schema + assertion rules for the mirror.