perf: pool map allocations in memkv List/ListDir operations

[abtreece]

Summary

• Use sync.Pool to reuse the seen map in List() and ListDir() operations
• Reduces allocations during high-frequency template processing (watch mode, short intervals)
• Add benchmarks for List/ListDir operations

Changes

• Add seenPool with initial capacity of 64 entries
• Modify List() and ListDir() to get/put maps from pool
• Use clear() builtin (Go 1.21+) to reset maps before returning to pool
• Add 6 benchmarks covering basic, high-frequency, and large store scenarios

Benchmark Results

BenchmarkList-8                      96932    11395 ns/op    6594 B/op    102 allocs/op
BenchmarkList_HighFrequency-8        37704    31726 ns/op   19526 B/op    306 allocs/op
BenchmarkListDir-8                  107212    11246 ns/op    6594 B/op    102 allocs/op
BenchmarkListDir_HighFrequency-8     78188    15330 ns/op    8002 B/op    126 allocs/op
BenchmarkList_LargeStore-8            9544   121462 ns/op   65845 B/op   1002 allocs/op
BenchmarkListDir_LargeStore-8        10000   118759 ns/op   65846 B/op   1002 allocs/op

Test plan

• All unit tests pass
• Full test suite passes with race detection
• Build succeeds
• Benchmarks run successfully

Fixes #449

[Copilot]

Pull request overview

This PR optimizes memory allocations in the memkv package by pooling the seen map used in List() and ListDir() operations. This reduces allocation overhead during high-frequency template processing scenarios like watch mode.

Changes:

• Introduced sync.Pool for reusing seen maps with an initial capacity of 64 entries
• Modified List() and ListDir() to acquire maps from the pool and return them after use
• Added comprehensive benchmarks covering basic, high-frequency, and large store scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/memkv/store.go	Added seenPool with sync.Pool and updated List()/ListDir() to use pooled maps
pkg/memkv/store_test.go	Added benchmark helper function and 6 benchmark functions for List/ListDir operations

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.81%. Comparing base (b08730e) to head (b21b392).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   59.84%   59.81%   -0.03%     
==========================================
  Files          48       48              
  Lines        5294     5305      +11     
==========================================
+ Hits         3168     3173       +5     
- Misses       1917     1923       +6     
  Partials      209      209              

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