docs: plan the road to 1.0 in cairn

[oddurs]

Problem

liam needs a plan a stranger can pick up. Without one, work starts in whatever order it occurs to someone, and the published speed budgets never turn into gates.

Approach

• cairn schema (cairn.toml), with project-specific types:
  • budget groups work under each of the five published promises (boot, listen, size, zero-drop, no-alloc).
  • spike closes with a written result that the items depending on it are handed.
  • Every feature template has a "Budget impact" section.
  • require_criteria is on, because a new backlog has no legacy of unticked boxes.
• 96 items: v0.1 to v0.4, v1.0, and later, each with acceptance criteria and real dependency edges. Unknowns are spikes. The kernel boot floor and the io_uring runtime choice come first.
• Decision 0051 narrows the zero-drop claim to what can be proven.
• Hook fix: post-merge no longer fails in a checkout without cairn.toml, which was the state of main when docs: add the CI status badge to the README #1 merged.

Budget impact

None. The plan defines the budgets and does not change the code.

Look at this sceptically

• v0.1 has 21 p0 items out of 24. Every one looked load-bearing for a usable MVP, but a container-first v0.1 would be smaller.
• No release has a date. Each ships when its gate passes.
• The hook fix has no automated test, because there is no shell test harness yet. It was checked by running the hook in a repository with cairn.toml and in one without.

Checks

• scripts/task check is green locally
• cairn check passes on all 96 items
• No attribution to an assistant anywhere in the commits or this description