docs: close the README with an overview instead of a stage section

[DavidCozens]

Pull request

What this stage adds

Every commit so far rewrote "## This stage —" to describe the one capability it
added. At the tip that section is the wrong shape: a reader arriving at the head
of the branch wants to know what the device ends up doing and what the whole
thing cost, not what the last commit changed about it.

It becomes "## Where it ends up" — the record the device actually sends, with its
four SD elements, mutual TLS and AES-256-GCM store; the generated total; and an
instruction to read the history forward from the Baseline commit rather than to
treat the tip as the product. That instruction is the point of the repository:
the diff is what a reader applies to their own build, so the sequence is the
deliverable and the tip is only where it happens to stop.

The STAGE-COST block stays where it was, so the headline figure is still written
by scripts/gen-cost-table.py from measurements/aes-gcm.csv. The minimum useful
cost is pointed at rather than quoted — it is already a row in the table below,
and a figure written into prose here would go stale silently the first time
anything moved.

No stage row, no CSV, and run-report.md is untouched: nothing here reaches the
binary, so there is no measurement to take.

Files

 README.md | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

Checklist

• The diff is application-only — no change to board bring-up, config headers, or build infra.
• measurements/docs-overview.csv committed. — not in this diff
• Row added to measurements/stages.tsv. — not in this diff
• README regenerated: python3 scripts/gen-cost-table.py.
• ./run.sh green (build + QEMU + baseline self-check) — run-report.md committed. — not in this diff

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Context for review: 21 of 21 sequential single-commit PRs replaying the integration on top of the Baseline root. Each lands green and reviewed before the next is built on it, because a change to an early commit would force every commit above it to be re-run and re-measured.

Summary by CodeRabbit

• Documentation
  • Updated the stage-3 documentation to describe the resulting RFC 5424 record format and delivery flow.
  • Documented mutual TLS delivery and persistence in a locally encrypted store when sending fails.
  • Clarified that the stages should be read as a step-by-step sequence from the Baseline commit.
  • Preserved the existing stage cost information.

[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: c9cbf478-d624-4aea-9a88-9583acc98f86

📥 Commits

Reviewing files that changed from the base of the PR and between 6e9896e and 16718bb.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

The README’s stage-3 narrative now describes the resulting RFC 5424 record, mutual TLS delivery, encrypted local spooling, and sequential interpretation from the Baseline commit. The existing Flash and RAM cost values remain unchanged.

Changes

README stage update

Layer / File(s)	Summary
Stage narrative and cost context README.md	Replaces the AES-GCM at-rest prose with final record, delivery, and encrypted spooling details while preserving the generated stage-cost values.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• cososo-ltd/solid-syslog-example#47: Updates overlapping README content for the AES-GCM at-rest stage.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately describes the README restructuring from a stage section to an overview.
Description check	✅ Passed	The description follows the template and covers the change, rationale, files, and checklist, though some checklist items are intentionally unfilled.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch stage/docs-overview

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