perf(parser): specialized fast path for default-option parsing

[evoactivity]

Applies the experiment-loop methodology (4 parallel agents on isolated copies, winners combined and measured stepwise) to the tree parser, which had fallen behind the SAX engine after the recent perf rounds.

What went in

• Thin parse() dispatcher: default-option calls route to a module-level parseRootFast with every option check constant-folded, pos as a true local (helpers inlined), and zero per-call closure allocation. Optioned calls take the untouched general path.
• Deferred ignorable whitespace: whitespace-only text recorded as integer triples; pure element containers drop it with no substring allocation, mixed content materializes lazily.
• Offset-based close-tag verification: flat range compare against the open tag's name offset — no substring().trimEnd() per close tag.
• Fused text scan + classification, integer loop guards, inline attribute-name scan with a name="value" fast path, entity-decode gated on & presence.

Measured and rejected

Parallel-array stacks, bounded JS pre-scan before SIMD indexOf, switch reordering.

Results

fixture	before	after	gain
small	1,367,666	1,687,050	1.23x
RSS	85,865	137,518	1.60x
SOAP	64,349	91,376	1.42x
Atom	35,517	52,689	1.48x
POM	13,759	20,580	1.50x
EPG	17,521	23,075	1.32x

Now 1.7-2.2x faster than tXml across fixtures. Confirmed in the browser demo (~318k → ~505k ops/s).

Verification

• 838 tests pass, typecheck clean, char-codes block regenerated via scripts/generate-char-codes.ts (idempotent)
• Parity harness: 7 fixtures × 8 option sets, identical trees and errors
• 30,000-case differential fuzz vs the previous parser (random malformed docs × 14 option sets): zero mismatches

Trade-offs

parseRootFast duplicates ~430 lines of parseChildren logic (semantic changes must be made in both; the parity gate + suite guard this), and module-level scratch buffers assume parse is never re-entered mid-call (holds today — no user callbacks run during tree building). BENCHMARKS.md DOM table and README claims regenerated.