docs: split T3 marks, add online-safety column, state the unix philosophy

[Kiran01bm]

Summary

docs/capabilities.md rendered every T3 row with the same ❌, so the page visually read as "pg-sprite doesn't support a bunch of things" when most of those rows are deliberate scope boundaries — operations that either have no online-safety problem to solve or belong to a different tool class. This PR makes the distinction visual and explicit, and states the underlying design stance — the Unix philosophy — loudly in the prominent positioning docs.

What

• T3 rows now carry one of three marks:
  • ⚪ no online-safety problem to solve — transactional catalog work or bootstrap on an object nothing reads yet; run it through owner tooling.
  • 🔵 a different tool class owns it — data-change runners, provisioning/IaC, convergence planners, expand/contract frameworks.
  • ❌ no online mechanism exists — PostgreSQL provides no online pattern to build on; the only rows where "unsupported" is the honest reading (EXCLUDE constraints, USING INDEX on a partitioned parent).
• Every matrix table gains an "Online-safety problem?" column: "Yes" for rows an online engine must solve; "No" cells name the tool class users should reach for.
• Tier-model section and the matrix legend updated to define the three marks.
• Unix philosophy called out explicitly — "do one thing, and do it perfectly", where the one thing is changing the shape of live PostgreSQL tables under concurrent load:
  • docs/vision.md: a dedicated paragraph after the intro, reconciling "one tool for all schema changes" with "do one thing" (depth across every table-shape change, never sprawl across object types), plus a framing line opening "What pg-sprite is not".
  • README.md: a paragraph after the planner positioning, pointing at capabilities.md.
  • docs/capabilities.md: a framing line in "What pg-sprite is — and why it exists" — the page is the map of where the one thing ends.

Why

The capabilities page is the doc we point users at for "does pg-sprite support this?". A scope boundary presented with the same mark as a hard limitation misrepresents the tool: only two rows in the whole matrix are genuinely impossible today, and the rest of T3 is design intent. Naming the Unix philosophy in the vision and README makes that intent legible before a user ever reaches the matrix, and the new column answers the follow-up question ("then what do I use?") inline.

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