feat(intent): postings reverses - red-storno reversal on a source void

[delchev]

Stacked on #6307 (transitions) - this branch contains that commit; review the top commit (9e1db5eb6a). The void transition is the event that triggers the reversal.

What

The postings glue could post a document when a source reaches a status, but not UN-post it: a voided/cancelled source left its journal entry standing. The new reverses: mode completes the pair:

postings:
  - name: invoicePosting
    event: { onTransition: Invoice, when: "Status == 3" }
    creates: JournalEntry
    backReference: Invoice
    rule: { entity: PostingRule, match: { documentType: "Invoice" } }
    items:
      - { Account: rule(receivableAccount), debit: "Net + Vat" }
      - { Account: rule(revenueAccount),    credit: "Net + Vat" }
  - name: invoiceStorno
    event: { onTransition: Invoice, when: "Status == 8" }   # the void transition (#6307)
    reverses: invoicePosting
    storno: Storno            # the created entity's to-one SELF-relation to the original

Semantics (red storno - the accounting correction convention)

• The reversal inherits creates/backReference/rule/map/items from the reversed sibling (declaring them is a validation error) and re-derives them from the source with every item amount expression negated on the SAME debit/credit side (Calc.eval("-(<expr>)", ...)) - never swapped sides, so turnover columns stay honest.
• It locates the original through the empty storno link (back-reference = this source, link null) and skips fail-soft when none exists - the source was never posted, nothing to reverse.
• Its creation stamps the storno link; both handlers' idempotency guards discriminate by it (the reversal counts only linked rows; the reversed sibling counts only unlinked ones - stornoProperty/stornoFilterProperty in the glue), so redeliveries of either event stay at-most-once.
• The reversal lands as a normal new document: DRAFT status init:, number placeholder, checks:, the review-and-Post task - corrections post into the open period.

Testing

• PostingsReversesIntentTest (parse + unknown-sibling / inherited-keys / missing-and-non-self-storno / storno-without-reverses rejections), GluePostingsReversesTest (negated expressions, inherited coordinates, storno keys on both entries) - full engine-intent suite green.
• IntentEmissionCoverageIT extended per the engine-intent testing contract: a Doc + PostDoc/VoidDoc transitions + docPosting/docStorno fixture; emission tokens (negation, fail-soft, storno stamp, sibling filter) and the full runtime chain (post a Doc → the balanced Entry appears; void it → the reversal appears with negative debit AND credit lines, linked to the original) - green locally.

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