fix: ensure route chunk names are relative instead of absolute paths

[alvadorn]

Symptom

With splitRouteModules enabled, routes using relative() without an explicit id emit absolute filesystem paths as part of their chunk names (e.g.,
/Users/<user>/...). This breaks build reproducibility, causes invalid asset URLs (double slashes), and leaks local path structures.

Root Cause

The plugin used route.id as the entry filename. While the manifest relativizes route.file, it keeps route.id absolute. When relative() is used, the id becomes
an absolute path, which the plugin then treated as a directory tree for the output.

The Fix

Introduced getRouteEntryBaseName(route, appDirectory) to derive both the main route entry and its chunks from a shared, relative base.

This shared helper also adds safety against:

• Path traversal: Collapses segments attempting to escape static/js/.
• Windows paths: Strips drive prefixes (e.g., C:\).

Behavior Change

• Cache-busting: Routes with explicit ids now use file-based names instead of id-based names. This is a rename, not a logic change.
• Deduplication: Routes pointing to the same file now share a single chunk entry.
• Collision prevention: Prevs collision between different files that previously shared the same id.

Tests

• Unit tests for naming rules (relative, absolute, traversal, and Windows).
• Integration tests reproducing the absolute path/double-slash symptom in a full production build.