fix: retain the source root when selecting a $defs entry so self-root refs resolve

[vishkaty]

Fixes #45.

What

validate --def <name> (and resolve --def) stack-overflowed when the selected $defs entry reaches a reference to the source file root. select_def built a wrapper that kept $schema, a $ref: "#/$defs/<name>" and the file's $defs, but dropped the file's $id, so a $ref: "#" inside the selected def resolved to the wrapper root — whose only content is the $ref back into the def — and recursed unboundedly. On the shipped payment_instrument.json, --def selected_payment_instrument aborts.

Fix

When (and only when) the selected def's reachable subgraph references the source root, the wrapper embeds the whole source file as an $id'd resource and roots the selection via <root_id>#/$defs/<name>, so #, escaping fragment pointers, and absolute self-$id refs all resolve to the real source root (the 2020-12 embedded-resource rule). Simply carrying $id onto the old wrapper does not work — # would still bind to the wrapper root and the cycle survives; the embedded resource is what gives # its intended target. When the file has no $id, a stable content-hashed URN is synthesized, guarded against colliding with any $id the source already declares.

Root-reference detection is scoped to the selected def's reachable subgraph (a visited-set walk following #/$defs/X edges from <name>), so a def with no root dependency of its own keeps the existing plain wrapper byte-for-byte, even when an unrelated sibling def in the same file carries a self-root ref. Transitive dependencies are still detected (a selected def that reaches the root through a def it references embeds correctly).

Scope (stated honestly)

This fixes the $ref: "#" self-root case that #45 reports. It does not fix two pre-existing, separate --def/bundler behaviors (present on main before this change, unchanged by it): a directly self-recursive #/$defs/<name> ref still overflows (the general bundler-recursion class, which overlaps #46), and a root-level $anchor ref reports fragment not found. Those are out of scope here.

Verification

• The #45 crash is gone on the real shipped schema: a complete selected payment instrument validates Valid; an incomplete one returns a clean "id" is a required property rejection; neither crashes. (Note: the exact payload in the issue omits the schema-required id, so post-fix that command correctly exits 1 with a validation error rather than 0 — the crash it demonstrated is fixed regardless of payload validity, since it occurred during schema construction.)
• New tests fail on the pre-fix code and pass after (self-root accept/reject, no-$id synthesis, the sibling-scope case, and a transitive-embed case), each kill-tested. Full suite 326 pass.
• resolve --def output for 8 unaffected defs is byte-identical to main. Derived resolve --op / validate --schema paths are untouched.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo build --release, and the pinned pre-commit are all clean.

Happy to adjust the approach if you prefer one of the alternatives.