[andreybolonin]

Preparing SCRIPT_FILENAME currently prepends / even when the request path already starts with it. The resulting extra slash makes path.Clean rebuild an otherwise clean path.

Only prepend / for unrooted paths. Keep path cleaning, the Windows locality check, and trailing-slash handling. Add cases for repeated leading slashes and traversal, plus a benchmark for rooted, relative, and traversal paths.

Earlier standalone measurements of this helper (Go 1.27.3, darwin/arm64, median of five 200ms samples):

Path	Before	After	Time reduction	Go B/op	allocs/op
/index.php	111.8 ns	69.69 ns	37.7%	80 → 32	4 → 1
/products/catalog/index.php	153.9 ns	110.9 ns	27.9%	144 → 48	4 → 1
Long rooted path	499.2 ns	413.3 ns	17.2%	656 → 176	4 → 1

These are helper-level results, not whole-request or RSS measurements. Relative paths had a 2.4% time reduction with unchanged allocations; traversal paths had an 11.0% reduction with Go B/op dropping from 96 to 72. Benchmarks were not rerun for this PR.

Validation:

• gofmt and git diff --check passed.
• The exact helper and its repository path tests passed with -race in an isolated harness without PHP linkage.
• A differential fuzz run against the original helper passed 553,791 executions.
• The isolated tests cross-compiled for Windows; Windows runtime tests were not run locally.
• Full FrankenPHP tests were not run: the local environment lacks the required PHP ZTS/shared-library and watcher setup.