Frontmatter YAML: inline " - " in a plain scalar throws ParseFailure (seq_item_ind) #73

Summary

The YAML frontmatter parser throws error.ParseFailure on a plain scalar value that
contains an inline " - " (space-hyphen-space). It mistokenizes the inline - as a
sequence-item indicator (seq_item_ind) and aborts, even though the hyphen is in the
middle of a value, not at the start of a line.

Minimal repro

---
description: prior notes - here.
---
body

$ zigmark --format frontmatter repro.md
(memory):2:28: error: unexpected token for 'key': .{ .id = .seq_item_ind, .loc = .{ .start = 26, .end = 28 } }
description: prior notes - here.
                           ^~
error: failed to parse frontmatter: error.ParseFailure
error: ParseFailure

Expected

description parses as the string prior notes - here.. In YAML, - is a
sequence-entry indicator only at the start of a node (line/indent); a hyphen inside a
plain scalar is ordinary text.

Isolation (what is / isn't the trigger)

• ✅ Trigger: inline " - " in a plain scalar → ParseFailure.
• ❌ Not parentheses: description: Aligned to the ISO 27701 (2025 revision). parses fine.
• ❌ Not multi-entry lists: a major_revisions: sequence of 2+ complete mappings parses
  fine (each entry's fields intact).

Secondary: memory leak on the error path

On the ParseFailure path the DebugAllocator reports leaked allocations:

error(DebugAllocator): memory address 0x... leaked:

Impact / how it surfaced

Found via dogfooding in sc2in/corp: policypress (which parses policy frontmatter through
zigmark.Frontmatter.initFromMarkdown) could not render a Privacy Policy whose
extra.major_revisions[].description contained " - ". Because a mis-parse drops fields,
it can also surface downstream as misleading validation errors (e.g. policypress
reporting major_revisions "missing the 'version' field" when every entry has one). A
policy revision history is a core ISMS use case, so value prose with " - " is common.

Environment: zigmark dev (built from current main via the flake), --format frontmatter.