feat: add --node flag to scope conversion to a single node

[tommymorgan]

Why

Large Figma files convert to hundreds of MB of optimized JSON, the entire
document, even when only one frame is needed. This adds a --node flag that
scopes conversion to a single node, given by its sessionID:localID (the id
Figma puts in file URLs; dash or colon form accepted). On a real 23MB .fig file this
turns a 313MB JSON export into ~1.5MB for one frame.

What

• --node <ID> on the CLI. Works for plain .fig and ZIP archives; for an
  archive it converts only the inner file containing the node, skips the rest,
  and errors clearly if no file matches. Rejected together with --raw.
• New convert_node(bytes, base_dir, target); convert() becomes a no-target
  wrapper, so existing callers are unchanged.
• The node is matched by its guid right after build_tree, before the
  transformation passes run, so only the targeted subtree is processed.
• NodeId is a u64 sessionID/localID pair that parses from either form via
  FromStr; a miss is reported as FigError::NodeNotFound(NodeId).

Usage

fig2json --help (the --node option is the addition):

Options:
  -o, --output <OUTPUT>   Output JSON file path (default: stdout) - Cannot be used with extract_dir
      --compact           Compact JSON output (default is pretty-printed with indentation)
  -v, --verbose           Verbose output for debugging
      --raw               Generate both transformed .json and raw .raw.json files (without transformations)
      --node <ID>         Extract only this node and its subtree (Figma node id, e.g. 6886:48774 or 6886-48774)
  -h, --help              Print help (see a summary with '-h')
  -V, --version           Print version

Example, pulling a single frame out of a locally-saved .fig (here a ZIP that
extracts to out/):

fig2json "design.fig" out/ --node 6886:48774
# writes out/<canvas>.json scoped to that node's subtree

Tests

• New schema::node_filter unit tests (id parsing: colon/dash/whitespace and
  rejected forms; subtree finding incl. sibling exclusion, first-match, and
  guarding against matching non-guid id keys), written test-first.
• CLI integration tests (tests/cli.rs) for --node and --raw validation.
• Full suite green (623 unit + 2 CLI + 67 doc), cargo clippy clean.