Use a slice to store section address for a field

[CascadingRadium]

• Currently, fieldsSectionsMap is of type []map[uint16]uint64, mapping fieldID → section → address.
• Since the section domain is a dense range from 0 to SectionCount, determined at process initialization, we can replace the inner map with a slice of length SectionCount.
• This avoids map overhead and reduces both memory usage and lookup cost.

[Copilot]

Pull request overview

This pull request optimizes memory usage and lookup performance for the fieldsSectionsMap data structure by replacing an inner map with a slice. The change leverages the fact that section IDs form a dense range from 0 to SectionCount.

Key Changes:

• Changed fieldsSectionsMap type from []map[uint16]uint64 to [][]uint64, eliminating map overhead
• Added SectionCount constant to track the total number of section types
• Refactored loadField method to return the field section map instead of accepting it as a parameter

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
section.go	Adds SectionCount constant to automatically track the total number of section types for sizing slice allocations
segment.go	Updates fieldsSectionsMap type definition, refactors loadField signature to return the section map, and initializes section maps as slices instead of maps
write.go	Removes obsolete FIXME comment about hard-coding section count, as the code now properly uses dynamic len(segmentSections)

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[abhinavdangeti]

I don't quite* follow the reason for this in this method - what're we using this for.

[CascadingRadium]

The earlier code used to take a map from the callee and populate it to make it part of the fieldsSectionsMap. But this is an issue as we have a map with an integer as a key, which invokes some overhead when reading, this PR aims to avoid this overhead as a micro optimization.