perf(start-client-core): O(1) buffer drain in client frame decoder

[anonrig]

What

Replace the O(n) bufferList.shift() in the client-side frame decoder (packages/start-client-core/src/client-rpc/frame-decoder.ts) with an O(1) head pointer.

extractFlattened() dropped each fully-consumed chunk from the front of bufferList with shift(). When a single large frame (e.g. a big RawStream payload) is assembled from many small network reads, the extract loop calls shift() once per chunk — and each shift() re-indexes the whole array, so reassembly degrades to O(n²).

This PR tracks the first un-consumed chunk with a bufferHead index and advances it in O(1) instead of shifting. readHeader() reads from bufferHead as well. Consumed slots are released for GC, and the array is compacted:

• fully drained (bufferHead === bufferList.length) → reset in O(1) (the common terminal state), or
• once the consumed prefix grows past a small threshold → splice() it off (amortized O(1) per consumed chunk).

This mirrors the existing index-pointer approach already used in transformStreamWithRouter for the same reason.

Why

Same hot path as the sibling zero-copy PR: decoding streamed server-function responses and RawStream payloads. The O(n²) bites specifically when one frame spans many buffered chunks.

Standalone micro-benchmark (Node, median of 12 runs), draining N buffered chunks:

Chunks	shift()	head pointer	Speedup
200	26.7 ms	2.5 ms	10.6x
1000	173 ms	15 ms	11.5x

Tests

• Existing frame-decoder suite passes.
• Added two tests for the changed paths:
  • a 200-byte CHUNK payload delivered one byte at a time (forces the header slow path + many whole-chunk consumptions + the fully-drained reset),
  • 100-byte frames fed in 7-byte reads that never align with frame boundaries, so the head pointer climbs past the compaction threshold repeatedly (exercises the splice() prefix drop).

test:unit   ✓ 20 passed
test:types  ✓ no errors
eslint      ✓ frame-decoder.ts clean (no new problems vs main)

Notes

• Independent of the sibling zero-copy PR. Both touch extractFlattened, so whichever merges second will need a trivial rebase.
• Pre-existing import/order lint errors in src/client/hydrateStart.ts (virtual-module imports) are unrelated to this change and present on main.

[anonrig]

Superseded by upstream PR TanStack#7663 (opened against TanStack/router).