feat(board): rank --plan on one score, with transitive chain-root credit

blocked-on was scanned and then used for a single display string; it never
reached the ordering. A blocked chain's root could therefore rank below the
repos waiting on it - measured on the real tree, the root opened at tab 14
while the two repos it held up opened at tabs 5 and 8.

--plan now ranks on one score instead of four hard buckets, because four
groups cannot express "this repo owes one message and releases two others":

    40 x repos released transitively + 15 x inbox
    +10 in-progress / +5 planned / +2 undeclared, +3 for a Sonnet row

Chain-root credit follows blocked-on transitively to the first repo that is
not itself blocked and credits only that root. Opening a blocked repo
releases nobody - its own next step is by definition waiting. A cycle, a
blocked-on naming an unscanned repo, and a blocked repo with no target all
credit NOBODY: inventing a root there yields a plan that looks correct and
sends the operator to the wrong repo.

Debt is deliberately uncapped. A cap was proposed alongside the credit and
declined: owing a reply is the other axis from a repo's own next step, and
answering is often what unblocks a chain. The weight carries that decision -
15 per message exceeds the sum of every other bonus (13), so one message
more still outranks any combination of status and cost. At the originally
proposed 12 a tiebreak could flip the debt order, which is the cap arriving
through the back door.

blocked-on becomes a record field, inserted BEFORE neste so no existing field
index moves: neste is free prose lifted from a STATE.md and may carry a
literal '|' from a markdown table, so it stays last.

why= now names the dominant term rather than the first rule that matched, so
a block says what opening it would release instead of repeating inbox:N.

Format is otherwise unchanged for both declared consumers: tab=, repo=, dir=,
why=, status=, neste=, command=/command_missing=, paste= and the fokus_*
header all keep their names and positions.

board-selftest 118 -> 134 (chain, cycle, dangling-edge and format fixtures
written first, all failing before the change).

CHANGELOG.md

@@ -5,6 +5,51 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.19.0] - 2026-08-02
+
+### Added
+
+- **Chain-root credit in `board.sh --plan`.** `blocked-on` was scanned and then
+  used for one display string (`blocked>X`); it never reached the ordering. For
+  every `blocked` repo the edge is now followed **transitively** to the root of
+  the chain — the first repo not itself blocked — and only that root is
+  credited with the repos it holds up. Opening a blocked repo releases nobody;
+  opening the root releases everything behind it. Measured on the real tree: the
+  root of the one live chain moved from tab 14 to tab 1, ahead of the two repos
+  waiting on it.
+- **`why=unblocks:N`.** `why=` now names the *dominant* scoring term rather than
+  the first rule that matched, so a block says what opening it would release.
+  Previously every planned block read `inbox:N` — the same sentence repeated.
+- **`blocked-on` is a record field**, inserted before `neste` so no existing
+  field index moves. `neste` is free prose lifted from a STATE.md and may carry
+  a literal `|` from a markdown table, so it stays last.
+
+### Changed
+
+- **`--plan` ranks on one score, not four hard buckets.** Four groups could not
+  express "this repo owes one message and releases two others", which is exactly
+  how a chain's root ended up below the repos waiting on it:
+
+      40 x repos released transitively + 15 x inbox
+      +10 in-progress / +5 planned / +2 undeclared, +3 for a Sonnet row
+
+  Every term is a lookup over a field the scan already read, so the order stays
+  reproducible and costs no model call.
+- **`--plan` header lines** now state the score and that a blocked repo never
+  receives credit. The format is otherwise **unchanged**: `tab=`, `repo=`,
+  `dir=`, `why=`, `status=`, `neste=`, `command=`/`command_missing=`, `paste=`
+  and the `fokus_*` header all keep their names and positions.
+
+### Not done, deliberately
+
+- **No cap on inbox debt.** Proposed alongside the credit and declined by the
+  operator: owing a reply is the other axis from a repo's own next step, and
+  answering is often precisely what unblocks a chain. The debt weight is 15 —
+  above the sum of every other bonus (13) — so one message more still outranks
+  any combination of status and cost, and the bonuses only separate repos that
+  owe the same. At the proposed 12 they could have flipped it, which would have
+  weakened the debt order through a tiebreak.
+
 ## [0.18.0] - 2026-08-02
 
 ### Added