perf(bridge-json): scan stream deltas instead of reparsing the buffer

[Nomadcxx]

Refs #139.

The bridge detector reparsed the whole candidate buffer on every partial delta, and called trimStart() on it every push. A streamed write envelope cost O(n²) on OpenCode's main thread.

Change

• Track bracket depth and string state per delta. Only run the full parse once the candidate is balanced outside a string, which any parseable envelope has to be.
• Check a fenced candidate's info line once, not on every push.

Proof

• New unit test: a 9 KB write streamed in 7-char chunks now hits JSON.parse once (was 175). Plus a fenced-envelope streaming test.
• Differential fuzz against the previous detector: 14,400 random chunkings (delta and snapshot modes, 6 tool sets, fences, arrays, prose, malformed input). Zero mismatches. A planted bug in the gate produced 3,064 mismatches, so the fuzz catches real divergence.
• Benchmark, 4-char deltas:

Envelope	Before	After
20 KB	26 ms	0.9 ms
50 KB	146 ms	1.8 ms
100 KB	588 ms	2.7 ms

• Live run: OpenCode 1.18.35 + local build, cursor-acp/auto. The model wrote a 40-function file through the bridge (call_bridge_* in the log), content intact including }]\" inside strings.
• bridge-json unit tests 55/55, opencode-loop integration 25 pass / 1 skip. Full-suite failures and tsc errors match main exactly (42 and 43, none in this file).