perf(jit): consolidate feedback and tier-state lookups

[huacnlee]

Issue # (if available)

Follow-up to merged #19; review this diff against main.

Description of changes

Consolidate observe_type and observe_call into one B-tree entry lookup, preserving occupied-at-capacity updates, rejected-entry accounting, exact feedback versions, and independent call-slot growth. Cache baseline tier state once after maintenance in record_hot; successful direct refresh returns before that cached value is reused.

Add three characterization tests covering the capacity, generation, version and megamorphic edges. Deliberate capacity and arity mutations fail the corresponding tests. Validation: 518 release runtime tests, 82 targeted ASAN/LSAN tests, workspace Clippy and formatting pass; independent code review found no issues.

Measured performance

Compared with a38bd01 (merged #18), the cumulative #19 + #20 changes improve generic-call Tier 1 speed by 19.3%, generic-call automatic by 18.1%, and recursive Tier 1 by 18.7%. The measured candidate is 99a1e78; the conflict-resolution merge retains that exact content tree. These are cumulative results, not the incremental gain of #20 over the current main containing #19.

The values below are steady-state latency converted to speed relative to the named baseline: 1.00x is parity; higher is faster. Intervals are paired 95% bootstrap confidence intervals.

Workload	Requested mode / actual execution	Speed vs. a38bd01	Speed improvement	95% interval
Generic calls	Tier 1	1.193x	19.3% faster	1.190x–1.197x
Generic calls	Tier 2 / actually Tier 1	1.163x	16.3% faster	1.160x–1.165x
Generic calls	Automatic / actually Tier 1	1.181x	18.1% faster	1.175x–1.185x
Recursive Fibonacci	Tier 1	1.187x	18.7% faster	1.183x–1.189x
Scalar loop	Tier 2	1.035x	3.5% faster	1.028x–1.038x
Scalar loop	Automatic	1.041x	4.1% faster	1.037x–1.047x

Controls and limitations:

• All direct-call modes, property-access modes, and interpreter comparisons are statistically tied. For example, direct-call Tier 2 ranges from 1.9% slower to 0.2% faster; property automatic ranges from 0.24% slower to 0.15% faster.
• Scalar Tier 1 is tied, between 2.3% slower and 0.6% faster. Recursive requested Tier 2 and automatic are also tied; automatic recursion has zero native entries and remains an interpreter-path control.
• The scalar-loop improvements have no isolated causal attribution. These measurements neither establish an overall no-regression result nor replace the complete clean-source acceptance matrix.
• Later rooted-property-guard changes and their measurements are outside this PR and are not included in this table.

Method: five discarded warmup processes, 30 fresh-process latency pairs per mode, ten throughput windows of at least one second per engine/mode, and 10,000 paired bootstrap resamples, pinned to CPU 0. Both runtimes use the identical corrected benchmark harness. Engine order is balanced independently of mode order (15/15 first positions for latency; 5/5 for throughput). Checksums, native entry/exit balance, and executable/source hashes were audited. The recorded runs use dirty-source diagnostic provenance; throughput windows count fresh workers including setup and warmup, not JavaScript operations.

Raw samples, exact driver/harness source, hashes, and intervals:
generic calls,
direct calls,
recursive Fibonacci,
scalar loop,
property access.

Archive raw evidence and a property profile: helper-frame validation remains the main sampled property cost. Document the next bounded ownership optimization and its safety constraints; property lowering itself is unchanged in this PR.

Checklist

• Added change to the changelog
• Created unit tests for my feature if needed