fix(pathfinder): crash on grid top/bottom-edge starts and goals

[Maudfer]

The bug

PathFinder.getValidNeighbors (src/app/game/PathFinder.ts) indexed the tile
matrix before any bounds check:

// TODO: handle grid edges
const neighborTile = matrix[neighbor.row]![neighbor.col];

Field.matrix is a { [row]: { [col]: Tile } } map, so for a neighbor off the
top edge (row === -1) or bottom edge (row === rows), matrix[row] is
undefined and indexing it throws TypeError: Cannot read properties of undefined (reading '<col>'). Out-of-bounds columns were already handled
gracefully (the col index just returns undefined → filtered), but rows
crashed. Any pathfinding request whose start or goal sits on the grid's
top/bottom edge (a very reachable state — roads/buildings can be placed against
the map border) crashed the pather.

The fix

Move the field.isValidPosition(row, col) bounds check ahead of the matrix
index so an out-of-bounds row (or col) is filtered before indexing — exactly how
out-of-bounds columns were already tolerated. The trailing isValid term in the
return is now guaranteed true, so it's dropped, and the stale
// TODO: handle grid edges is removed.

Tests

New test/pathFinder.test.ts (there was no pre-existing pathfinder test on this
branch) exercises real behavior — a valid collapsed path is returned, not merely
"doesn't throw":

• top edge: start on row 0 (NORTH neighbor is row -1) → routes across two footprints.
• bottom edge: start on the last row (SOUTH neighbor is off-grid) → routes across two footprints.
• every-neighbor-off-an-edge: a single road filling a 3×3 grid, corner start → returns the single collapsed step.

Revert-dance (proves the test guards the fix)

• Fix + test in place → npx jest test/pathFinder.test.ts green (3/3).
• git stash push -- src/app/game/PathFinder.ts (buggy source restored) → all 3 fail with TypeError: Cannot read properties of undefined (reading '0') at the old matrix[neighbor.row]! index line.
• git stash pop (fix restored) → green again.

Observed the fail→pass transition, so the test genuinely catches the regression.

Gates

• npx tsc --noEmit -p tsconfig.json — clean (exit 0).
• npx jest (full suite) — 73 suites / 647 tests green.
• Coverage on src/app/game/PathFinder.ts — 85.71% stmts / 75% branch / 100% funcs (≥ 80% stmt bar).

Notes on task vs. repo state

The task described a src/app/game/agents/ + test/agents/ layout with
--selectProjects agents, a per-module coverage project, an eslint gate, and a
pre-written pinned test. None of that exists on this branch: PathFinder.ts is
flat at src/app/game/, tests are a single flat test/ suite, there is no
eslint config/dependency in the repo, and no pathfinder test existed. I adapted
to the actual repo — fixed the real bug at the real path, added a new flat test,
and ran the gates that exist here (tsc / jest / coverage). Test imports use
../src/... to match all 73 existing test files (the repo convention; no test
uses path aliases).

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