* fix(slack): restore table block mode seam
Restore the shared markdown/config seam needed for Slack Block Kit table support, while coercing non-Slack block mode back to code.
* fix(slack): narrow table block seam defaults
Keep Slack table block mode opt-in in this seam-only PR, clamp collected placeholder offsets, and align fallback-table rendering with Slack block limits.
* fix(slack): bound table fallback rendering
Avoid spread-based maxima and bound Slack table fallback rendering by row, column, cell-width, and total-output limits to prevent resource exhaustion.
* fix(slack): keep block mode inactive in seam PR
Keep markdown table block mode schema-valid but runtime-resolved to code until the Slack send path is wired to emit table attachments.
* fix(slack): normalize configured block mode safely
Accept configured markdown table block mode at parse time, then normalize it back to code during runtime resolution so seam-only branches do not drop table content.
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