Optional capture in an enum branch crashes materialization

[zharinov]

An enum branch whose only capture is optional can match zero nodes. When it does, the branch produces no value, so the enum materializes as null. The debug type verifier then panics because the declared type is an enum but the value is null.

Repro

cargo run -p plotnik -- run -l javascript --compact \
  -q 'Q = (program [A: (identifier)? @id B: (number) @num] @item)' -s 'x'

Branch A is (identifier)? @id. The program's child is an expression_statement, not an identifier, so (identifier)? matches zero nodes. Branch A still succeeds with nothing captured.

Output (debug build):

BUG: Type and value do not match
item: type: enum, value: null

In a release build the verifier is a no-op, so this materializes a malformed value instead of panicking.

Expected

The output must not be a bare null where an enum value is declared. Two ways to fix it:

• an enum branch should not match on zero consumed nodes, or
• it should materialize { "$tag": "A", "$data": { "id": null } }, following the "every declared field is present, optional fields are null" policy.

Related

• Captured optional scope {...}? @cap double-nullifies inner fields #485, Captured optional {...}? @cap at a navigating child drops its struct scope (VM panics / corrupts output) #470 — same "optional matches zero, produces null where a struct is expected" family, but for captured optional scopes {...}? @cap rather than enum branches.