Story: Generalize as-of and as-of-bucket queries across repositories

Two distinct, general-purpose query shapes were built bespoke for market_observations_repository (curve-snapshot-builder-viewer task) and should be recognised and generalised as reusable patterns rather than staying one-off:

1. As-of: for a set of independently-updating rows sharing a key family (e.g. one row per point_id within a series_id), return the latest row per key at or before a given time — SELECT DISTINCT ON (key) ... WHERE ... < as_of ORDER BY key, time DESC=. Needs a supporting index ordered key-first, not time-first.
2. As-of buckets: N as-of snapshots at regular time intervals ending at a given boundary, computed as one SQL statement (generate_series() for the boundaries, a LATERAL subquery doing the per-bucket as-of reduction), not N round trips of the as-of query.

Distinct from the bitemporal "as-of" already documented in Time and Timestamps / temporal_composite_entity_versioning (valid_from < t < valid_to= — correction history for one entity instance). This pattern is about multiple independently-ticking rows sharing a key family, reconstructing a snapshot across a set. compute_telemetry_repository::latest_node_samples() already used the identical shape for per-host latest telemetry before this story existed — the shape recurs, but nobody has named or captured it as reusable until now.

• The as-of and as-of-bucket query shapes are documented as reusable patterns (reference implementation: market_observations_repository::read_as_of()/read_as_of_buckets()).
• Either a shared ores.database helper exists generating the parameterised SQL for both shapes, or a documented decision exists to defer the helper until a second concrete caller needs it, keeping the reference implementation as the copy-from source in the meantime.
• The "buckets are one DB query, not a C++ loop over N as-of calls" principle is captured as a hard rule somewhere discoverable (a knowledge doc or repository-layer convention doc), not left to be rediscovered per repository.

• Building the shared helper immediately if only one real caller exists — avoid premature abstraction from a single caller (see Acceptance).

• projects/ores.marketdata/core/src/repository/market_observations_repository.cpp — read_as_of()=/=read_as_of_buckets(), the reference implementation this story is distilled from.
• projects/ores.marketdata/modeling/ores.marketdata.market_observation.org — the observations_series_point_datetime_idx index entry.
• projects/ores.compute/core/src/repository/compute_telemetry_repository.cpp — latest_node_samples(), an earlier, independent instance of the same shape.

• Curve snapshot builder/viewer — the task this was raised from.
• Time and Timestamps — the bitemporal (transaction-time) "as-of", a different but related concept.