FOLLOW-UP: Extract shared utility functions from source and runtime packages (from PR #215)

[adnaan]

Context

This follow-up task was identified during the review of PR #215.

Source PR: #215
PR Title: feat: add computed source type for derived/aggregated data
Suggested by: Code reuse review

Task Description

Several utility functions are duplicated between internal/source/computed.go and internal/runtime/expr.go:

• getField / getFieldValue — case-insensitive field lookup on row maps
• toFloat64 / tryFloat64 — numeric type conversion with different coverage
• sumValues / sumField — field aggregation with divergent error handling
• applyFilter / filterData — row filtering with different comparison semantics

These have subtly different behavior (e.g., computed.go skips non-numeric silently while expr.go returns errors). A shared internal/fieldutil package would consolidate these and ensure consistent behavior.

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This issue was automatically created by prmonitor from PR review comments.