fix(ui): calculator polish — smart input steps, unified tech/MAT lock idiom, tech floor, speed-lock ceiling fix

- pkg/calc: DriveForSpeed treats restMass==0 as a valid ceiling-only
  case (every positive drive solves it), so locking the displayed
  speed of a D=1, W=A=S=C=0 ship is no longer a phantom "infeasible".
- ship-design-area: drive/weapons/shields/cargo inputs use a JS-driven
  smart step on ArrowUp/ArrowDown (0↔1 jump, otherwise ±0.1) and hide
  the native spinner so it cannot produce invalid (0, 1) values;
  armament keeps its native step 1.
- Tech and planet MAT cells follow the same lock idiom as goal-seek
  locks: open padlock (🔓) over the inherited value → click to open
  an input with a closed padlock (🔒). The padlock slot is always
  reserved, so the column width is stable.
- Tech overrides (design area and modernization target) are floored
  at the player's current tech on this turn — a lower value is
  flagged as invalid.

ui/docs/calculator-ux.md

@@ -24,8 +24,18 @@ in as a per-ship result rather than a separate mode.
 
 1. **Ship Class design area** — five blocks (drive, armament, weapons,
    shields, cargo) and four tech levels (drive, weapons, shields,
-   cargo). Tech defaults to the player's current tech and shows a lock
-   icon once overridden; clicking it resets to the default.
+   cargo). Tech defaults to the player's current tech: the cell renders
+   the inherited number with an open padlock; clicking the open lock
+   activates an input (closed padlock), where the player may type an
+   override at or above their current tech. Clicking the closed
+   padlock resets to the default. The padlock slot is always reserved,
+   so the column width does not shift as the lock state toggles. The
+   four ship-class blocks (drive, weapons, shields, cargo) use a smart
+   keyboard step that respects the engine value rule (`0` or `≥ 1`):
+   ArrowUp from 0 jumps straight to 1, otherwise +0.1; ArrowDown from
+   1 collapses to 0, otherwise −0.1, never producing an invalid value
+   in `(0, 1)`. The native spinner is hidden on these inputs (it would
+   produce invalid intermediates); armament keeps its native step 1.
 2. **Calculator area** — derived results: empty/loaded mass, empty/
    loaded speed, attack, defence, bombing (per ship), cargo capacity.
    A load toggle (empty / full / custom) sets the cargo load (in cargo
@@ -35,17 +45,27 @@ in as a per-ship result rather than a separate mode.
    the load is pinned to empty and the toggle is disabled.
 3. **Planet area** — when an own planet is selected on the map, shows
    its MAT (overridable) and the single-turn build rate (ships per turn,
-   turns per ship). The realistic multi-turn forecast with CAP/COL
+   turns per ship). The MAT follows the same lock idiom as the tech
+   cells: the planet number renders with an open padlock, clicking
+   opens an input with a closed padlock, and the closed padlock resets
+   to the planet value. The realistic multi-turn forecast with CAP/COL
    supply is planned (see ../ROADMAP.md).
 
 ## Locks and goal-seek
 
-Two distinct lock semantics share one icon (a closed padlock; it only
-appears once a value is pinned, click to release):
+Two distinct lock semantics share one padlock affordance. Both follow
+the same idiom — an open padlock (🔓) means *value is inherited /
+derived, click to override*; a closed padlock (🔒) means *value is
+pinned by the player, click to reset*:
 
 - **Override locks** on inputs that have a default — the four techs and
-  the planet MAT. Editing one overrides the default; the lock resets it.
-  Any number may be overridden at once.
+  the planet MAT. By default the cell shows the inherited number plus
+  an open padlock; clicking it switches to an input plus a closed
+  padlock for typing the override. Closing (clicking the closed
+  padlock) resets to the default. Any number may be overridden at once.
+  Tech overrides are floored at the player's current tech on this
+  turn — a lower value is flagged as invalid. The same floor applies
+  to the modernization target tech.
 - **Goal-seek locks** on derived results. Pinning a result back-solves
   the single input it claims, which then renders read-only (computed):
 
@@ -60,12 +80,16 @@ appears once a value is pinned, click to release):
 
   Only **one** result may be locked at a time (the others' lock
   affordances disable with a tooltip). An unreachable target — e.g. a
-  speed at or above the stripped-hull ceiling `20 × driveTech`, or a
+  speed above the stripped-hull ceiling `20 × driveTech`, or a
   solved block that fails the value rules — leaves the locked cell in a
   red error state and does not apply. Inverse solving lives in
   `pkg/calc/solve.go`; the bisection for defence → shields is the only
   non-analytic case. Locking a speed is disabled when the drive block is
-  zero (a deliberately immobile ship has no speed to back-solve).
+  zero (a deliberately immobile ship has no speed to back-solve). With
+  the drive block as the only non-zero mass the displayed speed equals
+  the ceiling exactly (every positive drive gives the same speed), so
+  the solver accepts that ceiling target as a feasible lock and any
+  positive drive solves it.
 
 ## Validation and display