Task: Generic unified-diff history renderer in ores.shell

Re-enable the shell's history-diff command, disabled since the currency pilot's hand-written shape was reverted, as one generic, entity-parameterised unified-diff renderer that consumes the generic history.v1.get response. Context lines come from unchanged fields; +/- pairs come from the server-computed changes. All comparison happens server-side (ores.history's dispatch_registry + per-entity history provider); the shell renderer is pure formatting, shared by every entity's history-diff command.

• [X] ores.history restructured to the api/core layout used elsewhere in the codebase.
• [X] Generic render_history_diff() added in ores.shell, consuming the generic history.v1.get subject and formatting the server-computed diff as unified-diff text (context lines + -/+ pairs, or full addition from /dev/null for the oldest version).
• [X] Currency's history command merged into one entry point: flat table by default, --diff for the unified diff, --diff --version <n> to pick a version (rejected without --diff). Replaces the earlier separate history-diff command.
• [X] Country wired the same way: registered as a second history.v1.get dispatch provider (it already had a codegen'd country_history_field_mapper and get_country_history) and its shell history command merged into --diff=/–version=.
• [X] Manually verified both against live entities with real history (add/rename, table view, diff view, diff of the initial version, and the --version-without=–diff= guard).
• [ ] Rolled out to the remaining entities with an existing plain history shell command: tenants, accounts, change_reasons, change_reason_categories. These four do not yet have a codegen'd *_history_field_mapper or a confirmed get_*_history service method — that's prerequisite codegen work, not just wiring.
• [ ] Rolled out to the ~22 other entities that don't have any shell history command yet (out of this task's original "for every entity" scope; likely belongs to a follow-on task given the codegen prerequisite above).
• [X] Shell recipes updated (doc/recipes/shell/currencies/history_2.org, doc/recipes/shell/countries/history_3.org) to document --diff=/–version=; the standalone "History Diff" recipe/script for currencies was retired.

• Split ores.history into api/ (protocol types, version_builder) and core/ (registrar, dispatch_registry, handler), matching the api/core layout already used by ores.refdata and siblings.
• Add ores.shell::app::commands::render_history_diff(): takes the dispatch key (entity_type_of()), entity id, and an optional version; queries history.v1.get over the NATS session and prints the response as a unified diff.
• Re-enable currency's history command (previously a disabled history-diff) by delegating to the new generic renderer instead of the removed hand-written currency_version_history shape.
• Merge it into the plain history command rather than keeping a separate history-diff: history <id> keeps the flat table by default, --diff switches to the unified diff, --diff --version <n> picks a version. Retired the standalone recipe/script.
• Verified manually against a freshly provisioned tenant: added/edited a test currency, checked table view, diff view, diff of the oldest version (all-additions from /dev/null), and the --version without --diff guard.
• Remaining: repeat the "delegate to render_history_diff()"' wiring for the other 26 entities' history commands.

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Restructured ores.history into the api/core layout and added a generic ores.shell::app::commands::render_history_diff() that renders any entity's history.v1.get response as a unified diff — context lines from unchanged fields, -/+ pairs from the server-computed changes, or every field as an addition from /dev/null for the oldest version.

Wired currency and country onto it: both registered as history.v1.get dispatch providers in ores.refdata's registrar, and both entities' shell command merged from a separate history-diff into history <id> [--diff] [--version <n>] (--version rejected without --diff). Manually verified end-to-end against a live provisioned tenant for both entities: table view, diff view, diff of the initial version, and the --version-without=–diff= guard.

Also discovered and filed two follow-ups rather than folding into this task's scope:

• A pre-existing bug where an entity's version resets after a soft-delete + recreate cycle, so read_all's version DESC ordering and version_builder's position-based renumbering can diff two unrelated "generations"' rows together (task 2A05DA93).
• The remaining entities with a plain shell history command (tenants, accounts, change_reasons, change_reason_categories) lack the codegen'd *_history_field_mapper=/=get_*_history that currency and country already had; noted on the "Shell entity commands — top-level commissioning story" capture as the natural place to fold --diff rollout into once that story's history facet is templatized, rather than hand-wiring the remaining ~26 entities here.