Cache unchanged templates during IDE editing via TemplateContext

[johanrd]

• Introduces TemplateContext, a per-template cache slot that persists across rewriteModule() calls
• When a template is unchanged between keystrokes, both preprocess() and templateToTypescript() are skipped entirely
• VirtualGtsCode holds a TemplateContext[] passed through the pipeline — one slot per template position, no unbounded growth
• CLI path passes no contexts — zero overhead, no regression

Benchmarks

ts-template-imports-app test app — JS edit (template unchanged)

File	Templates	Lines	Before	After	Speedup
Playground.gts	3	62	762µs	327µs	2.3x
Greeting.gts	2	23	215µs	138µs	1.6x
augmented.gts	1	15	81µs	51µs	1.6x
with-declaration-consumer.gts	1	19	129µs	43µs	3.0x
colocated/index.gts	1	14	99µs	68µs	1.5x

Speedup scales with template size. On a production app with larger templates on a M1 mac:

File	Templates	Template lines	JS/TS lines	Before	After	Speedup
a.gts	1	1335	539	287.7ms	5.7ms	50x
b.gts	1	510	147	33.1ms	1.5ms	22x
c.gts	1	325	764	18.6ms	5.0ms	4x
d.gts	16	446	440	13.6ms	4.9ms	3x

Memory

After 1000 keystrokes: 1 slot for a single-template file. No growth.

CLI (ember-tsc)

No TemplateContext is passed in the CLI path — all cache checks are skipped. No regression.

Test plan

• 144/144 transform tests pass (140 existing + 4 new cache hit/miss tests)
• Verify IDE responsiveness on a large .gts file
• Verify ember-tsc CLI produces identical output