docs(spike): prioritize next operation and envelope fixtures

[Markodiba]

Summary

Answers the "which Stellar operation and envelope types should be added next" spike with:

• A coverage matrix of 19 candidate operation/envelope types against threat relevance, user exposure, decoder complexity, determinism, consumer support, and test value
• A threat mapping tying each candidate to a concrete product warning
• Two prioritization strategies (threat-first vs. cost-adjusted ROI) compared side by side
• A recommended first tranche: multi-op envelopes, setOptions (signer takeover), manageSellOffer, accountMerge, memo coverage, and malformed-envelope fixtures
• Fixture-format prototypes and an 8-step implementation sequence

Adds docs/spikes/operation-envelope-priority.md. No fixtures, scores, or XDR files changed.

Type of change

• Fixture change — adds, removes, or rescores a destination or asset
• Transaction change — adds or removes a sample XDR
• Documentation only — README, CONTRIBUTING, CHANGELOG prose, etc.
• CI / tooling

Changelog

Skipped — documentation-only PR, no fixture files (destinations.json, scores.json, transactions/*.xdr) touched.

Score-regression guard

• N/A — no scores were changed.

Validation

• npm run validate passes locally.
• npm test passes locally (46/46).
• node scripts/check-secrets.mjs passes locally.

🤖 Generated with Claude Code

[Markodiba]

Superseded — opened against upstream instead: Gryd-lock#85