[annotator] admit type_assertion expressions (class-only, #658 step 3)

[cs01]

Before

Type assertions (x as Foo) were never admitted to the annotator's typeOf cache. Every as Foo expression fired a cache miss at codegen time, forcing resolveExpressionTypeRich to recompute a type that's literal in the AST.

After

Annotator admits type_assertion expressions when:

• assertedType passes isSafelyAnnotatable (no unions, nullable, function types, or inline object shapes)
• Resolved type has sourceKind === "class" and passes isSafeVariableAnnotationType

Interface-kind assertions intentionally excluded — an earlier attempt to admit them broke stage 2 self-hosting (same arch-divergence class as #662 member_access). Re-evaluate after Tier 3 #11 (interface concretization pass).

Description

Follows data posted in #658 (comment 2026-04-22): divergence tracer showed 1249 codegen-time miss events for type_assertion sourceKind=interface on chad-native compile, zero hit-diff events anywhere. Class-only subset is the narrowest safe admission. ~25 LOC.

Verified with full npm run verify (stage 2 self-hosting green).

Gate-loosen table

Kind	Class	Interface
variable	✅ #660	✅ #661
member_access	✅ #668	❌ blocked (Tier 3 #11)
type_assertion	✅ this PR	❌ blocked (Tier 3 #11)

[github-actions]

Benchmark Results (Linux x86-64)

Benchmark	C	ChadScript	Go	Node	Place
Cold Start	1.0ms	0.9ms	1.2ms	27.5ms	🥇
Fibonacci	0.814s	0.762s	1.561s	3.196s	🥇
Hash Map Lookup	0.094s	0.063s	0.092s	0.112s	🥇
Regex Match	0.017s	0.005s	0.022s	0.005s	🥇
Binary Trees	1.502s	1.290s	2.735s	1.150s	🥈
File I/O	0.117s	0.088s	0.086s	0.196s	🥈
JSON Parse/Stringify	0.035s	0.051s	0.174s	0.135s	🥈
N-Body Simulation	1.695s	2.119s	2.211s	2.386s	🥈
SQLite	0.052s	0.371s	0.493s	0.451s	🥈
Matrix Multiply	0.493s	0.592s	0.613s	0.378s	🥉
Monte Carlo Pi	0.389s	0.410s	0.404s	2.248s	🥉
Quicksort	0.215s	0.247s	0.213s	0.262s	🥉
Sieve of Eratosthenes	0.016s	0.026s	0.018s	0.039s	🥉
String Manipulation	0.008s	0.018s	0.017s	0.037s	🥉

CLI Tool Benchmarks

Benchmark	ChadScript	grep	node	xxd	Place
Hex Dump	0.566s	—	0.968s	0.130s	🥈
Recursive Grep	0.020s	0.011s	0.108s	—	🥈