perf(mojo): pre-size 2048-byte n=1 buffers and faster strings

[leo-gan]

Summary

• Official n=1 writer capacity 256 → 2048 (payloads are 411–491 bytes)
• Re-vendor gld-json Error fixes #8: single-scan string write, ASCII decode, reserved lists
• Document items / event attrs reserved

Official all-single vs #143 (median skip warmup):

cell	#143 ser/des	now ser/des	Ember	vs Ember
message n=1	258 / 484	256 / 418	248 / 540	0.97× / 1.29×
document n=1	2144 / 1563	555 / 1273	502 / 1615	0.90× / 1.27×
strings n=1	2257 / 4835	658 / 2894	595 / 1147	0.90× / 0.40×
event n=1	786 / 1841	369 / 1144	331 / 949	0.90× / 0.83×
message n=100	16576 / 39155	16303 / 32895	19613 / 31738	1.20× / 0.96×

Fidelity 1.0. 7/20 official cells now beat Ember. 1.5× floor still not met.

Validation

• mojo test_roundtrip.mojo ok
• all-single json stem 2026-09-09-150051