feat(lang): 27.3 - a reducer on a table's row: reducer T in S.table on A, @Target, Row<T>, @RowVerb, @Column

An action names one row of a ludic.base Table<T> by its handle, in a field marked @Target, and
`reducer Deer in Herd.deer on Spook(r: mut Row<Deer>, n: Noise, a: Spook)` runs once, for that row
alone (the table may sit down a path, S.w.tab). The drain resolves the handle (tb_row) and hands the
reducer a Row<T> - new in ludic.base: tb, row, h, rec - that the queue keeps, one per row reducer,
filled in place, so a targeted action allocates nothing; a stale handle runs nothing, and
LUDIC_ACTIONS_LOG=1 prints a line for it (@alloc_ok). Row reducers order among an action's by their
state's name, then the table's path.

Checked at compile time (actions_rows.ludic): the row reaches r.rec and r.h only - r.tb / r.row
refused, the view never assigned, stored, copied or handed on except to a @RowVerb (a function of
the record's own module taking Row<T> first; any other function taking a row is refused); a field
marked @Column (a table column mirrors it) is not written through r.rec; only the module owning the
state declares a row reducer; one @Target, an int, per action; the states between the row and the
action are read. `mut` is allowed on a Row<T> parameter.

ludic schema's code section gains row_reducers (record, table, state, action, target, predicted,
net, module, at) and row_verbs (name, record, module, at), and every action its target; row
reducers are left out of `reducers`. ludic deps and ludic-lsp name a row reducer
`reducer Deer in Herd.deer on Spook`. vocab: @Target, @Column, @RowVerb; docs/language pages;
LANGUAGE.md "A reducer on a row"; examples actions/rows and ten rejects; test.ludic feat, reject and
schema cases (not run); changes/row-reducers.md. Reseeded; bootstrap-cfree fixpoint holds (307497
lines); Maroon Lake's `ludic build --check` is clean against this tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

changes/row-reducers.md

@@ -0,0 +1,23 @@
+bump: minor
+type: feat
+**A reducer on a table's row** (phase 27.3). An action names one row of a ludic.base `Table<T>` by
+its handle, in a field marked `@Target`, and `reducer Deer in Herd.deer on Spook(r: mut Row<Deer>,
+n: Noise, a: Spook)` runs once, for that row alone (the table may sit down a path, `WildState.w.tab`).
+
+- **The drain resolves the handle** (`tb_row`) and hands the reducer a `Row<T>` (new in ludic.base:
+  `tb`, `row`, `h`, `rec`) that it keeps, one per row reducer, filled in place: a targeted action
+  allocates nothing. A stale handle runs nothing; `LUDIC_ACTIONS_LOG=1` prints a line for it. Row
+  reducers order among an action's reducers by their state's name, then the table's path.
+- **The row goes no further**, checked at compile time: a row reducer reaches `r.rec` and `r.h`, not
+  `r.tb` or `r.row`; it does not assign, store, copy or hand on the row except to a `@RowVerb` (a
+  function of the record's own module taking `Row<T>` first); a field marked `@Column` (a table
+  column mirrors it) is written only by a verb; only the module owning the state declares a row
+  reducer; states between the row and the action are read. `mut` is allowed on a `Row<T>` parameter.
+- **`ludic schema`'s code section** gains `row_reducers` (`record`, `table`, `state`, `action`,
+  `target`, `predicted`, `net`, `module`, `at`) and `row_verbs` (`name`, `record`, `module`, `at`),
+  and every action its `target`; row reducers are not listed under `reducers`. `ludic deps` names a
+  row reducer `reducer Deer in Herd.deer on Spook`; ludic-lsp's outline does too.
+- Examples `actions/rows`; rejects for a missing `@Target`, two of them, a `@Column` write, reaching
+  the table, a row handed to a non-verb, a table that is not one, the parameter shape, a second
+  state written, a verb with no row, and a reducer outside the state's module. `@Predicted`, `@Net`
+  and `@Broadcast` on actions and row reducers are later steps of phase 27.