A scenario header with no body passes validate, then archive refuses the change #1857

Description

A requirement whose only scenario is a header with nothing under it gets
opposite verdicts from the two commands that check it:

• openspec validate <change> reports the change valid.
• openspec archive <change> refuses it, with a generic "Requirement must
  have at least one scenario" that does not name the requirement.

This is the validate/archive disagreement the scenario-parity work was meant to
rule out: the failure is deferred from authoring time to archive time.

Reproduction

In a project created with openspec init + openspec new change edit, with a
valid proposal and tasks, the delta openspec/changes/edit/specs/billing/spec.md:

## ADDED Requirements
### Requirement: Credit Notes
The system SHALL issue a credit note when an invoice is voided.

#### Scenario: Invoice voided

openspec validate edit --json   # "valid": true
openspec archive edit --yes     # exit 1
#   ✗ Requirement must have at least one scenario
#   Aborted. No files were changed.

Control: the same scenario with - **WHEN** ... / - **THEN** ... lines under
it validates and archives.

Expected

validate rejects what archive will reject, and names the requirement, so
the author fixes it while writing the delta.

Actual

validate passes; archive fails later without naming the requirement.

Root cause

The two scenario counters the code says must agree do not:

• The delta path counts every non-fenced #### header:
  SCENARIO_HEADER (src/core/parsers/requirement-text.ts:30) and
  countScenarios (:94), used by validate for ADDED and MODIFIED
  (src/core/validation/validator.ts:272, :307).
• The spec path keeps a scenario only when its body has content:
  if (scenarioSection.content.trim())
  (src/core/parsers/markdown-parser.ts:176). Archive validates the rebuilt
  spec on this path.

The parity requirement is written at requirement-text.ts:26: "so the delta
counter must too (parity)".

Other shapes that disagree the same way: a scenario body of only blank or
whitespace lines, and a closed header (#### Scenario: X ####) with no body.

Environment

• OpenSpec 1.13.0 (reproduced on the published npm package and on a build of
  main @ 9d4e597)
• Node v20.19.0 and v22, Linux