feat: avoid padded-grid allocation in table exports

[npkanaka]

What

Add an opt-in, record-oriented Markdown table exporter and a CPU-only serving image built from this repository. Table exports retain actual values without allocating a padded rectangular grid.

Why

Wide, sparse tables can use excessive memory during export. Iterating existing cells avoids padding missing positions while retaining explicit empty fields, coordinates, header flags and merged ranges.

Thought

Keep the structured document model and other export formats unchanged. Enable the exporter through a process-local startup factory so normal, worker and reload processes use the same behavior. Build the converter and adapter from the checkout, with a pinned runtime and installed-source verification.

Alternatives considered

• Converting CSV tables into plain text would discard the structured table model.
• Changing the SDK's default exporter globally would affect callers that did not opt in. A dedicated serving integration keeps that boundary explicit.

Blindspots

• Local execution covers native ARM64 and emulated AMD64. Native execution on both architectures is configured in CI but has not yet been verified remotely.
• End-to-end conversion checks cover CSV and HTML, not PDF model behavior or concurrent large-upload capacity.
• Exported Markdown can be larger than a compact table. Downstream consumers still need appropriate storage and request-size limits.
• Functional tests do not establish vulnerability remediation or production readiness.
• GHCR publication and package permissions have not been exercised. Publication rebuilds the tested source revision rather than promoting identical local test-image binaries.

ELI5

Read the filled squares in a spreadsheet instead of creating every empty square in a huge rectangle.

Test plan

• 19 unit tests, including no-grid access, caption placement, rich-cell heading behavior, merged cells and unchanged document JSON.
• CSV and HTML HTTP conversion in default, worker and reload modes on native ARM64 and emulated AMD64.
• Large synthetic CSV conversion in all three modes on ARM64, comparing all returned values and record labels with the source.
• Worker identity, unexpected restart and OOM-counter checks.
• Installed converter version/source verification and unfiltered dependency checks.
• Ruff lint/format, Actionlint and staged whitespace checks.
• Confirm native AMD64 and ARM64 CI checks.
• Verify GHCR publication and the resulting multi-platform manifest after review and merge.

See integrations/stackgen_serve/README.md for build commands and limits.

[github-actions]

AI triage (advisory)

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Warning

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CI safety: needs maintainer care before you approve CI.

• .github/workflows/stackgen-serve.yml: changes CI workflows, actions, or CI scripts
• integrations/stackgen_serve/build.sh: adds or changes a shell script
• integrations/stackgen_serve/test-image.sh: adds or changes a shell script

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