Cache repeated computations in program construction hot paths

[no-yan]

Part of a series reducing allocation churn on hot paths (see #63895 for
motivation and series-wide measurements).

This PR

• fileLoader.toPath: called once per import edge with heavily
  repeated file names; each call re-normalized and re-lowercased the
  path. Cache the canonical path per loader (collections.SyncMap).
• Resolver conditions: newResolutionState rebuilt the same
  conditions slice for every resolution. Precompute the import/require
  variants once per resolver; project-reference redirects (rare, options
  differ from the resolver's own) fall back to GetConditions. The
  import/require classification is factored into conditionsUseImport,
  shared with GetConditions, so the two cannot drift.

Results

Allocation site	Before	After
fileLoader.toPath → ToFileNameLowerCase	81k–105k	4k
GetConditions	44k	0

(allocations per full vscode/src check, heap-profile attribution.)
BenchmarkNewProgram is unchanged (±1%, p > 0.19).

Verification

• Full vscode/src output is byte-identical to main (43 errors).
• TestGetConditionsCacheMatchesGetConditions asserts the cached slices
  equal GetConditions for every resolution mode across representative
  option sets (node16/nodenext/bundler, custom conditions,
  noDtsResolution).

🤖 Generated with Claude Code

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