[Kinfe123]

Streamfold kept completion records for every descendant after its JSON container completed. Drop those redundant records when marking a field complete: the parent completion flag already answers descendant queries, while replacing the field still invalidates its completion state.

Adds contracts for completed nested containers while siblings stream, array-index aliases, duplicate-key replacements, special keys, negative replacement numbers, and immutable snapshot history. Adds a reproducible retained-heap benchmark without exposing parser internals.

Validation:

• All six GitHub CI jobs passed, including Chromium, Firefox, WebKit, Node 20 core tests, and the Assistant UI demo.
• pnpm check: 178 JavaScript tests and 11 Rust tests, plus type, syntax, Rust formatting, and clippy checks.
• Chromium smoke test, package dry-run, and frozen-lockfile installation passed.
• pnpm bench, pnpm bench:sdk, and snapshot A/B validation against 0.1.14 passed; the snapshot comparison asserts every partial value. No CPU speedup is claimed.
• Memory benchmark compares published 0.1.14 with this change in fresh Node 24.21.0 processes on Apple M1: three alternating trials, eight warmups, explicit GC, 16 active streams, 2,000 nested records per stream, 217,801-byte prefixes, 4,096-character chunks.
• Immutable mode retained JavaScript heap: 92.36 MiB → 18.83 MiB while the array is open; 92.34 MiB → 10.99 MiB once the root completes. Live mode shows the same reduction. The values and field states are asserted in each run.
• Run pnpm bench:completion-memory /path/to/streamfold-0.1.14/src/index.js to reproduce.

These are synthetic JavaScript heap measurements, excluding WASM linear memory; this PR makes no production latency claim. The runtime change is internal and does not change the public API.