JS-984 Add centralized FsCache layer for file system operations

[vdiez]

Summary

• Introduce a new FsCache singleton in the shared package that consolidates file system caching across the codebase
• Migrate sourceFileContentCache to use FsCache for file contents (keeping parsed AST cache separate)
• Migrate tsconfigContentCache to use FsCache with separate tracking for missing-but-fallback tsconfig files
• Update IncrementalCompilerHost to use FsCache and track known content per host instance for proper change detection

This is the first step toward unifying all FS-related caches into a single source of truth.

Architecture

┌─────────────────────────────────────────────────────────────────┐
│                     Analysis Layer                               │
│  (analyzeWithProgram, analyzeWithIncrementalProgram, rules)     │
└─────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│                    Program Cache                                 │
│  (ProgramCacheManager - ts.Program instances)                   │
└─────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│                  Parsed Content Cache                            │
│  - parsedSourceFileCache (ASTs)                                 │
│  - programOptionsCache (parsed tsconfig options)                │
│  - dependenciesCache (parsed package.json deps)                 │
└─────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│                      FS Cache Layer (NEW)                        │
│  Single source of truth for all file system access:            │
│  - File contents (source, tsconfig, package.json, etc.)        │
│  - File existence / directory listing                           │
│  - Invalidation based on baseDir change or fsEvents            │
└─────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│                    File System (node:fs)                         │
└─────────────────────────────────────────────────────────────────┘

Test plan

• All program tests pass (110 tests)
• tsconfigs tests pass (5 tests)
• TypeScript type checking passes

Future work

The following caches can be migrated in subsequent PRs:

• Package.json reading (MinimatchCache and related)
• File discovery caches (closestPatternCache, patternInParentsCache)
• Unified invalidation logic across all caches

🤖 Generated with Claude Code

[hashicorp-vault-sonar-prod]

JS-984