docs: add error_codes.md aligned with on-main errors.rs

[graveyieldprotocol]

Summary

• Adds docs/error_codes.md as the canonical markdown source-of-truth for both programs' error tables, sourced verbatim from programs/grave-scanner/src/errors.rs (6000-6019) and programs/grave-vault/src/errors.rs (7000-7014).
• Updates docs/README.md with a new Reference section linking the new file.
• Documents the drift in a dedicated section: Combined Tech Doc v3.0 .docx §4.2 lists a different scheme (7000-7018 with names like AMMRedemptionFailed, JupiterSwapFailed, InvalidMerkleProof, InvalidSnapshotData, ComputeBudgetTooLow). On-main is the lock per docs/README.md's "markdown wins when .docx disagrees" rule; the .docx scheme is superseded.

Why this PR

Spec drift between code and canon docs is treated as a bug per docs/README.md. The drift on error tables wasn't catchable until now because the Combined Tech Doc has no markdown source-of-truth in the repo — only the frozen .docx snapshot. This PR introduces a focused, narrow-scope markdown file for the error tables alone so future drift is detectable by inspection.

A follow-up PR can add a scripts/check-error-codes.sh CI step that diffs errors.rs discriminants against error_codes.md rows. Out of scope here since this is a no-code docs-only PR.

Out of scope

• Full Combined Tech Doc markdown migration (docs/grave-scanner-grave-vault-combined.md). Separate, larger PR if/when ordered.
• Regeneration of published/GraveScanner_GraveVault_CombinedTechnicalDocumentation_v3_0.docx. Per docs/README.md, that happens at the next minor revision.
• Drift-detection CI script.
• m5/m6/m7 GraveVault work (parallel workstream; new error codes 7015+ will append in this file as those PRs land).

Heads-up on CI

The anchor-build job has been failing on main since PR #15 (toolchain investigation in progress, parallel workstream). This docs-only PR will inherit the red status — merge is gated on the parallel CI fix landing first per the locked security posture (no merging while required checks are red).

Test plan

• terminology-lint.sh local pre-check — passes (no forbidden vocabulary in either file)
• Markdown lints cleanly (verified locally)
• CodeRabbit review (mandatory pre-merge)
• anchor-build job green (gated on parallel CI fix landing first)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added comprehensive error codes reference covering scanner (6000–6019) and vault (7000–7014) program error conditions with detailed specifications.
  • Updated main documentation index with link to new error codes reference.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9de688a4-a903-4dd2-beb8-42fabda027e7

📥 Commits

Reviewing files that changed from the base of the PR and between 10a0fa2 and 87cbf9f.

📒 Files selected for processing (2)

• docs/README.md
• docs/error_codes.md

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📝 Walkthrough

Walkthrough

A new docs/error_codes.md file establishes the authoritative single source of truth for GraveScanner (6000–6019) and GraveVault (7000–7014) error codes, declaring a mirroring contract to Rust errors.rs sources with a "Rust sources win" rule and lock-step maintenance requirement. The docs/README.md is updated to add a Reference section linking to this new document.

Changes

Error Codes Reference Documentation

Layer / File(s)	Summary
Error codes reference document docs/error_codes.md	Establishes the contract declaring error_codes.md as the living source of truth mirroring programs/grave-scanner/src/errors.rs and programs/grave-vault/src/errors.rs with a "Rust sources win" rule. Specifies the lock-step update convention and on-chain error code derivation from Anchor's #[error_code] discriminants. Publishes the GraveScanner error table (6000–6019) and GraveVault error table (7000–7014), each mapping numeric codes to named errors and failure conditions. Documents how this markdown supersedes the v3.0 .docx snapshot and sets the requirement that future combined tech docs reference this file rather than re-tabulating codes. Includes a footer noting the mirroring date.
README documentation index docs/README.md	Adds a new "Reference" section with a table entry linking to "Error codes (Scanner 6000-6019, Vault 7000-7014)" and its corresponding error_codes.md document.

🎯 1 (Trivial) | ⏱️ ~3 minutes

A reference carved in stone (or markdown, at least),
Error codes gathered, documented with finesse—
GraveScanner, GraveVault, now clear and precise,
The source of truth established, once and twice. 🪶✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly describes the main change: adding error_codes.md aligned with errors.rs sources, which is the primary objective.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/error-codes-realign

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