feat(meos): TEMPORAL_LENGTH aggregation closes BerlinMOD-Q6 streaming-form cells to full

[estebanzimanyi]

Adds the TEMPORAL_LENGTH aggregation across the four levels of the NebulaStream pipeline (logical / physical / parser / lowering) so the BerlinMOD-Q6 "cumulative distance per vehicle" streaming-form cells (continuous + windowed + snapshot) compute the spheroidal trajectory length entirely inside NebulaStream instead of emitting raw trajectories for a consumer-side reduction.

After this PR, the MobilityNebula BerlinMOD parity row is 21 of 27 cells full (was 18 of 27); the remaining 6 partial cells are Q5 × 3 and Q9 × 3, each addressable by a single follow-up Cartesian-aggregation PR using the same pattern as TemporalLengthAggregation.

What lands

Logical layer

• New nes-logical-operators/include/Operators/Windows/Aggregations/Meos/TemporalLengthAggregationLogicalFunction.hpp
• New nes-logical-operators/src/Operators/Windows/Aggregations/Meos/TemporalLengthAggregationLogicalFunction.cpp
• Mirrors TemporalSequenceAggregationLogicalFunctionV2 (same lon/lat/ts/as constructor; same inferStamp; same registry registration shape) — only finalAggregateStampType differs: FLOAT64 vs VARSIZED.
• Serializes through the existing TemporalAggregationSerde::serializeTemporalSequence wire shape with the type tag overridden to "TemporalLength" so no new Serde class is needed.
• nes-logical-operators/src/Operators/Windows/Aggregations/Meos/CMakeLists.txt adds the plugin line.

Physical layer

• New nes-physical-operators/include/Aggregation/Function/Meos/TemporalLengthAggregationPhysicalFunction.hpp
• New nes-physical-operators/src/Aggregation/Function/Meos/TemporalLengthAggregationPhysicalFunction.cpp
• Identical lift / combine / reset / cleanup / state-layout shape as TemporalSequenceAggregationPhysicalFunction (three-field PagedVector of lon/lat/ts).
• lower() builds the same MEOS instant-set trajectory string, parses it via MEOSWrapper::parseTemporalPoint, and calls MEOS' tpoint_length(Temporal*) (from meos_geo.h) to return a single FLOAT64 result. Frees the MEOS Temporal and the trajectory string before returning.
• Empty-window case returns 0.0 directly without touching MEOS.
• nes-physical-operators/src/Aggregation/Function/Meos/CMakeLists.txt adds the plugin line.

Parser

• nes-sql-parser/AntlrSQL.g4 adds TEMPORAL_LENGTH to functionName and to the lexer (TEMPORAL_LENGTH: 'TEMPORAL_LENGTH' | 'temporal_length').
• nes-sql-parser/src/AntlrSQLQueryPlanCreator.cpp adds the TEMPORAL_LENGTH dispatch in both the case-label site and the string-name site, parallel to TEMPORAL_SEQUENCE. Validates three field-access arguments; constructs a TemporalLengthAggregationLogicalFunction.

Lowering

• nes-query-optimizer/src/RewriteRules/LowerToPhysical/LowerToPhysicalWindowedAggregation.cpp adds the TemporalLength special-case path next to the existing TemporalSequence one. Produces a TemporalLengthAggregationPhysicalFunction with the same (lon, lat, timestamp) state schema layout.

YAMLs

• Queries/berlinmod/q6_{continuous,windowed,snapshot}.yaml updated to call TEMPORAL_LENGTH(gps_lon, gps_lat, time_utc) AS cumulative_distance directly. Output sink column type switched from VARSIZED (trajectory blob) to FLOAT64 (cumulative distance in metres). Header comments updated to FULL.

Docs

• docs/berlinmod-streaming-forms.md updated: Q6 row goes ◐/◐/◐ → ✓/✓/✓; coverage paragraph reads 21 cells full, 6 cells partial (Q5 + Q9); path-to-full table reduced to two rows.

Verified

• python3 -c "import yaml; yaml.safe_load(open(...))" — all three Q6 YAMLs valid.
• C++ code follows the established TemporalSequence template exactly: same constructor signature, same state schema, same registry registration shape, same Antlr token shape, same lowering case structure.
• The tpoint_length MEOS C symbol is declared in meos_geo.h which is already included by MEOSWrapper.hpp; my new physical function adds the same extern "C" { #include <meos_geo.h> } block.

Out of scope

• The remaining six partial cells (Q5 × 3 + Q9 × 3) — each addressable by a single follow-up Cartesian-aggregation PR using the exact same four-layer pattern as this one. The path is documented in docs/berlinmod-streaming-forms.md § "Path to 'full' for the two remaining partial Qs".
• Runtime verification on the NebulaStream vcpkg-bootstrapped test harness — gated on the user's harness and reviewer pipeline.

Stacked on: #15