perf(router-core): use the simple interpolatePath lane on the client

[anonrig]

Summary

Split out of #8085 so bundle size and performance can be measured independently.

The $param / $ scan already matches the braced parser for templates without {...}. Link-shaped to/params navigation can use it without waiting for isServer. Braced segments still go through parseSegment.

Test plan

• packages/router-core/tests/path.test.ts
• packages/router-core/tests/optional-path-params.test.ts
• packages/router-core/tests/optional-path-params-clean.test.ts

Summary by CodeRabbit

• Bug Fixes
  • Improved path interpolation consistency across server and client environments.
  • Plain paths and parameterized segments such as $param continue to resolve as expected.
• Performance
  • Streamlined simple path handling for faster route resolution.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 72b45a70-852d-4df1-a72f-9726f7fd1cf7

📥 Commits

Reviewing files that changed from the base of the PR and between f97188f and 854ab0a.

📒 Files selected for processing (1)

• packages/router-core/src/path.ts

Included review availability: Your plan includes up to 10 reviews per rolling hour; 8 remain after this review.

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📝 Walkthrough

Walkthrough

The path interpolation fast path no longer depends on the server environment. The implementation discards the internal server option while preserving parameter tracking, encoding, missing-parameter handling, splat aliases, and trailing-slash behavior.

Changes

Path interpolation

Layer / File(s)	Summary
Enable the interpolation fast path in all environments packages/router-core/src/path.ts	The plain-segment interpolation path now runs without the server-only guard. The internal server option is explicitly ignored. Existing interpolation behavior remains unchanged.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 854ab

The change routes eligible client navigation through a simpler path interpolation lane while retaining braced-segment parsing, with targeted tests listed; no actionable merge-blocking risk remains beyond normal checks and review.

Possibly related PRs

• TanStack/router#7977: Both changes modify interpolatePath in packages/router-core/src/path.ts, but this PR removes the server-only guard while that PR changes substring calls to slice.

Suggested labels: package: router-core

Suggested reviewers: sheraff

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: enabling the simple interpolatePath lane for client-side router-core performance.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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