Add temporary hit points as a separate damage buffer

Temp HP absorbs damage before current HP, cannot be healed, and
does not stack (higher value wins). Displayed as cyan +N after
current HP with a Shield button in the HP adjustment popover.
Column space is reserved across all rows only when any combatant
has temp HP. Concentration pulse fires on any damage, including
damage fully absorbed by temp HP.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

specs/003-combatant-state/spec.md

@@ -21,6 +21,7 @@ interface Combatant {
   readonly initiative?: number;       // integer, undefined = unset
   readonly maxHp?: number;            // positive integer
   readonly currentHp?: number;        // 0..maxHp
+  readonly tempHp?: number;           // positive integer, damage buffer
   readonly ac?: number;               // non-negative integer
   readonly conditions?: readonly ConditionId[];
   readonly isConcentrating?: boolean;
@@ -96,6 +97,19 @@ As a game master, I want HP values to survive page reloads so that I do not lose
 Acceptance scenarios:
 1. **Given** a combatant has max HP 30 and current HP 18, **When** the page is reloaded, **Then** both values are restored exactly.
 
+**Story HP-7 — Temporary Hit Points (P1)**
+As a game master, I want to grant temporary HP to a combatant so that I can track damage buffers from spells like Heroism or False Life without manual bookkeeping.
+
+Acceptance scenarios:
+1. **Given** a combatant has 15/20 HP and no temp HP, **When** the user sets 8 temp HP via the popover, **Then** the combatant displays `15+8 / 20`.
+2. **Given** a combatant has 15+8/20 HP, **When** 5 damage is dealt, **Then** temp HP decreases to 3 and current HP remains 15 → display `15+3 / 20`.
+3. **Given** a combatant has 15+3/20 HP, **When** 10 damage is dealt, **Then** temp HP is fully consumed (3 absorbed) and current HP decreases by the remaining 7 → display `8 / 20`.
+4. **Given** a combatant has 15+5/20 HP, **When** 8 healing is applied, **Then** current HP increases to 20 and temp HP remains 5 → display `20+5 / 20`.
+5. **Given** a combatant has 10+5/20 HP, **When** the user sets 3 temp HP, **Then** temp HP remains 5 (higher value kept).
+6. **Given** a combatant has 10+3/20 HP, **When** the user sets 7 temp HP, **Then** temp HP becomes 7.
+7. **Given** no combatant has temp HP, **When** viewing the encounter, **Then** no extra space is reserved for temp HP display.
+8. **Given** one combatant has temp HP, **When** viewing the encounter, **Then** all rows reserve space for the temp HP display to maintain column alignment.
+
 ### Requirements
 
 - **FR-001**: Each combatant MAY have an optional `maxHp` value (positive integer >= 1). HP tracking is optional per combatant.
@@ -120,6 +134,15 @@ Acceptance scenarios:
 - **FR-020**: The HP area MUST display the unconscious color treatment (red) and the combatant row MUST appear visually muted when status is `unconscious`.
 - **FR-021**: Status indicators MUST NOT be shown when `maxHp` is not set.
 - **FR-022**: Visual status indicators MUST update within the same interaction frame as the HP change — no perceptible delay.
+- **FR-023**: Each combatant MAY have an optional `tempHp` value (positive integer >= 1). Temp HP is independent of regular HP tracking but requires HP tracking to be enabled.
+- **FR-024**: When damage is applied, temp HP MUST absorb damage first. Any remaining damage after temp HP is depleted MUST reduce `currentHp`.
+- **FR-025**: Healing MUST NOT restore temp HP. Healing applies only to `currentHp`.
+- **FR-026**: When setting temp HP on a combatant that already has temp HP, the system MUST keep the higher of the two values.
+- **FR-027**: When `maxHp` is cleared (HP tracking disabled), `tempHp` MUST also be cleared.
+- **FR-028**: The temp HP value MUST be displayed as a cyan `+N` immediately after the current HP value, only when temp HP > 0.
+- **FR-029**: When any combatant in the encounter has temp HP > 0, all rows MUST reserve space for the temp HP display to maintain column alignment. When no combatant has temp HP, no space is reserved.
+- **FR-030**: The HP adjustment popover MUST include a third button (Shield icon) for setting temp HP.
+- **FR-031**: Temp HP MUST persist across page reloads via the existing persistence mechanism.
 
 ### Edge Cases
 
@@ -131,7 +154,10 @@ Acceptance scenarios:
 - Submitting an empty delta input applies no change; the input remains ready.
 - When the user rapidly applies multiple deltas, each is applied sequentially; none are lost.
 - HP tracking is entirely absent for combatants with no `maxHp` set — no HP controls are shown.
-- There is no temporary HP in the MVP baseline.
+- Setting temp HP to 0 or clearing it removes temp HP entirely.
+- Temp HP does not affect `HpStatus` derivation — a combatant with 5 current HP, 5 temp HP, and 20 max HP is still bloodied.
+- When a concentrating combatant takes damage, the concentration pulse MUST trigger regardless of whether temp HP absorbs the damage — "taking damage" is the trigger, not losing real HP.
+- A combatant at 0 currentHp with temp HP remaining is still unconscious.
 - There is no death/unconscious game mechanic triggered at 0 HP; the system displays the state only.
 - There is no damage type tracking, resistance/vulnerability calculation, or hit log in the MVP baseline.
 - There is no undo/redo for HP changes in the MVP baseline.