Task: Retire per-entity history dialog templates for the generic HistoryDialog

Implement History Diff Architecture end to end, replacing every one of the 67 *HistoryDialog classes (the 6 hand-rolled ones migrated onto HistoryDialogBase earlier in this story, plus the 61 codegen-generated ones this task was originally scoped to) with a single, non-templated HistoryDialog widget. This supersedes the original "adopt HistoryDialogBase in the template" scope: rather than migrating per-entity generated dialogs onto a shared base class, the per-entity dialog classes and their generating templates are retired outright.

Was gated on the codegen org-model migration: that migration (Codegen unified model — org-mode migration) completed 2026-06-11 — the gate is clear. This task additionally depended on Redesign server-side history-diff architecture (DONE 2026-07-11) for the design it implements.

• One generic NATS history subject (history.v1.get) exists, with a server-side dispatch table keyed by entity_type_of(); every entity's typed get_<entity>_history subject is removed.
• Every entity has a codegen'd, plain (not hand-written, not generic- reflection) field-to-string mapper, registered as a history provider.
• ores.diff::diff_entry carries old_spans=/=new_spans; the test coverage list in the redesign task's Notes is satisfied.
• HistoryDialogBase and all per-entity *HistoryDialog classes are deleted, along with cpp_qt_history_dialog.{hpp,cpp}.mustache and qt_history_dialog_ui.mustache; a single HistoryDialog widget replaces them, taking (entity_type, entity_id).
• Open=/=Revert dispatch through a codegen'd per-entity action registry (client side); no typed domain payload flows through HistoryDialog signals.
• ores.shell's disabled currencies history-diff command is replaced by one generic entity-parameterised shell renderer, re-enabled for every entity.
• History responses carry no full domain payload; open/revert fetch the typed entity by id/version through each entity's existing typed request when needed.
• Zero-diff regeneration checks green for whatever codegen facets remain (field mapper, history-provider/action registrations); no facet still emits a per-entity dialog class.

1. ores.diff: add diff_span to diff_entry; implement span computation in engine::compute (prefix/suffix for single-line, line-LCS + per-line token diff for multiline); cover the test list from the redesign task's Notes.
2. Entity identifier: add the entity_type_of() codegen facet, generating the free-function trait per entity.
3. Server side: add the per-entity field-mapper codegen facet; introduce the generic history.v1.get subject and its dispatch-table handler (reapplying Entity-composed registrars); register each entity's mapper as a history provider; remove the per-entity get_<entity>_history subjects and handlers as each entity migrates.
4. Qt: build the single HistoryDialog widget (version list, async load, toolbar, changes-tab rendering with diff_span colour highlighting per the GitHub-style capture); add the codegen'd per-entity Open=/=Revert action registry; wire callers (list windows/controllers) to construct HistoryDialog(entity_type, entity_id) instead of a typed dialog subclass.
5. Retire the old shape: delete HistoryDialogBase, every per-entity *HistoryDialog class, and cpp_qt_history_dialog.{hpp,cpp}.mustache=/=qt_history_dialog_ui.mustache once every entity has migrated; zero-diff guardrails confirm no facet still emits them.
6. Shell: replace the disabled currencies history-diff command with the generic entity-parameterised renderer, re-enabled for every entity.
7. Composite entities: wire the "Details" affordance for child-driven versions (per the architecture doc's composite section) — opens another HistoryDialog instance with the child's (entity_type, entity_id).

Given the size, this likely wants splitting into sub-tasks by phase (ores.diff spans; server dispatch; Qt widget; shell; retirement sweep) once implementation starts — revisit before diving in.

Split into six finer-grained sibling tasks, one per Plan phase, before any implementation started — this task's own scope was too large to track as a single unit of work:

1. ores.diff: intra-value diff spans
2. Codegen the entity_type_of() identifier trait
3. Generic server-side history dispatch: one subject, per-entity handlers
4. Single generic HistoryDialog widget in ores.qt
5. Retire HistoryDialogBase, per-entity dialogs, and their templates
6. Generic unified-diff history renderer in ores.shell

Dependency order: (1) and (2) are independent and can run in either order; (3) needs both; (4) and (6) each need (3); (5) needs (4) and (6) — it retires the old shape only once nothing still depends on it. This task closes with no code changes; its Plan and Acceptance were distributed into the six tasks above.