[ty] Store interpolated string elements in boxed slices

[charliermarsh]

Summary

Store f-string and t-string elements as a boxed slice after parsing instead of retaining a growable Vec.

The collection is never extended after construction. Boxing it discards spare capacity and reduces the retained collection handle from 24 to 16 bytes on 64-bit targets.

Extracted from #27591.

[astral-sh-bot]

Memory usage report

Summary

Project	Old	New	Diff	Outcome
flake8	40.38MB	40.36MB	-0.06% (23.23kB)	⬇️
trio	93.28MB	93.24MB	-0.04% (34.85kB)	⬇️
sphinx	164.92MB	164.86MB	-0.04% (66.70kB)	⬇️
prefect	453.68MB	453.48MB	-0.05% (209.47kB)	⬇️

Significant changes

Click to expand detailed breakdown

flake8

Name	Old	New	Diff	Outcome
parsed_module	15.50MB	15.48MB	-0.15% (23.23kB)	⬇️

trio

Name	Old	New	Diff	Outcome
parsed_module	23.25MB	23.22MB	-0.15% (34.85kB)	⬇️

sphinx

Name	Old	New	Diff	Outcome
parsed_module	34.30MB	34.24MB	-0.19% (66.70kB)	⬇️

prefect

Name	Old	New	Diff	Outcome
parsed_module	89.10MB	88.90MB	-0.23% (209.47kB)	⬇️

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.