html.t27 and xml.t27: share 11 helper bodies so their #5084 fixes can land and leave the seal ledger #7438

The #5084 versions of specs/tri/encoding/html.t27 and specs/tri/encoding/xml.t27 pass with master's t27c (10/10 and 11/11 on the t27c lab). They cannot land as-is, because they copy 11 helper bodies between them and tools/dupe_scan.py goes red.

The copied helpers are bad_attr, byte_order_mark, bytes_are, digit_of, from_hex, hex_value, put_byte, put_code, refused_hex, same (template.t27 has a third copy) and skip_space.

Done when:

• these helpers live in one spec that both parsers use;
• both specs are resealed on the lab;
• their 4 stale lines are dropped from tools/seal_baseline.txt.

Watch the use traps first (#7281, #7176). Found while clearing the ledger in #7386.

Boundary

• specs/tri/encoding/html.t27
• specs/tri/encoding/xml.t27
• tools/dupe_scan.py
• tools/seal_baseline.txt

User Scenarios

• Given html.t27 and xml.t27 each contain copies of the same 11 helper bodies, When tools/dupe_scan.py runs, Then it reports duplication and exits non-zero.
• Given a new shared spec containing the 11 helpers, When both html.t27 and xml.t27 use that spec and are resealed, Then tools/dupe_scan.py passes and the seal ledger has no stale entries for these specs.

Requirements

• FR-001: A new spec file MUST be created containing the 11 helper bodies (bad_attr, byte_order_mark, bytes_are, digit_of, from_hex, hex_value, put_byte, put_code, refused_hex, same, skip_space).
• FR-002: specs/tri/encoding/html.t27 MUST use the new shared spec and remove its local copies of the 11 helpers.
• FR-003: specs/tri/encoding/xml.t27 MUST use the new shared spec and remove its local copies of the 11 helpers.
• FR-004: Both specs MUST be resealed on the t27c lab so their seals reflect the shared helpers.
• FR-005: The 4 stale lines referencing html.t27 and xml.t27 MUST be removed from tools/seal_baseline.txt.

Success Criteria

• tools/dupe_scan.py runs and exits with exit 0
• t27c lab reseal of specs/tri/encoding/html.t27 completes with exit 0
• t27c lab reseal of specs/tri/encoding/xml.t27 completes with exit 0
• grep -c 'stale.*html.t27\|stale.*xml.t27' tools/seal_baseline.txt returns 0
• The new shared spec file exists and is used by both html.t27 and xml.t27