feat: WritableFS, LocalFS, MemFS, and fs.FS-backed FileSystemLoader

[panyam]

Summary

Adds portable filesystem abstractions to Templar, enabling template storage beyond the local disk (S3, IndexedDB/WASM, in-memory for tests).

New types

Type	What it is
WritableFS	Interface: fs.FS + WriteFile/MkdirAll/Remove/Rename
LocalFS	WritableFS backed by local OS filesystem (relative to root dir)
MemFS	WritableFS backed by in-memory map (testing, WASM)

FileSystemLoader enhancement

New FileSystems []fs.FS field where FileSystems[i] backs Folders[i]:

// Old way (unchanged, backward compatible):
loader := NewFileSystemLoader("/usr/share/templates")

// New way (portable):
loader := &FileSystemLoader{
    Folders:     []string{"templates", "layouts"},
    FileSystems: []fs.FS{myMemFS, myS3FS},
    Extensions:  []string{"html"},
}

When FileSystems[i] is nil (or the slice is shorter), falls back to OS filesystem — fully backward compatible.

Motivation

Slyds (presentation toolkit) needs to store decks on various backends. Rather than invent a separate filesystem abstraction, we're adding it to Templar where the template loader can use it directly. This benefits any Templar user who wants templates from non-OS sources.

Tests

19 new tests covering LocalFS, MemFS (including write isolation, directory inference), FileSystemLoader with MemFS, mixed FS+OS, backward compatibility.

All existing tests pass unchanged.