perf(inode): optimize lookupCount struct layout to 8-byte scalar

[kislaykishore]

Description

This PR optimizes the lookupCount helper in internal/fs/inode by converting it from a 24-byte composite struct ({id fuseops.InodeID, count uint64, destroyed bool}) into an 8-byte scalar int64 (where -1 denotes the destroyed state). Inode IDs are passed dynamically to Inc(id) and Dec(id, n) for panic formatting instead of being retained inside each inode instance.

Memory Savings & Go Heap Allocator Class Boundary

• Crucially, FileInode shrinks from 304 B to 288 B, crossing a Go runtime heap allocator size class boundary (320 B -> 288 B).
• Saves 32 bytes (-10.0%) of real heap memory per file inode, yielding 320.0 MB direct heap RSS savings per 10M file inodes.
• Shrinks dirInode, SymlinkInode, and baseDirInode by 16 bytes each.

Struct / Type	Baseline Size	Optimized Size	Struct Delta	Go Heap Class	Heap Allocation Delta	RAM Saved (per 1M files)	RAM Saved (per 10M files)
lookupCount	24 B	8 B	-16 B (-66.7%)	Stack/Internal	—	—	—
FileInode	304 B	288 B	-16 B (-5.3%)	320 B -> 288 B	-32 B (-10.0%)	32.0 MB	320.0 MB
dirInode	240 B	224 B	-16 B (-6.7%)	240 B -> 224 B	-16 B (-6.7%)	16.0 MB	160.0 MB
SymlinkInode	152 B	136 B	-16 B (-10.5%)	160 B -> 144 B	-16 B (-10.0%)	16.0 MB	160.0 MB
baseDirInode	152 B	136 B	-16 B (-10.5%)	160 B -> 144 B	-16 B (-10.0%)	16.0 MB	160.0 MB

Benchmark Performance

Measured via benchstat (n=10, p < 0.05):

Benchmark Target	Master Baseline	Optimized Branch	CPU Latency Delta	Baseline Memory	Branch Memory	Allocation Delta
LookupCount_Inc	2.161 ns/op	1.881 ns/op	-12.93% (p=0.000)	0 B/op, 0 alloc	0 B/op, 0 alloc	0.0%
LookupCount_Dec	2.340 ns/op	2.285 ns/op	-2.35% (p=0.043)	0 B/op, 0 alloc	0 B/op, 0 alloc	0.0%
Alloc_FileInode	130.9 ns/op	129.3 ns/op	~0.0% (parity)	320 B/op, 1 alloc	288 B/op, 1 alloc	-10.0% B/op

Link to the issue in case of a bug fix.

N/A

Testing details

1. Manual - Executed make build and verified formatting/linter with 0 issues.
2. Unit tests - Executed unit tests: go test -v -race ./internal/fs/inode/....
3. Integration tests - N/A

Any backward incompatible change? If so, please explain.

N/A

[gemini-code-assist]

Code Review

This pull request refactors the lookupCount helper in internal/fs/inode/lookup_count.go from a struct to a custom int64 type, representing a destroyed state with a value of -1. This change simplifies the type and removes the need for an explicit initialization step, instead requiring the inode ID to be passed directly to the Inc and Dec methods. Corresponding updates were made across various inode implementations (base_dir.go, dir.go, file.go, and symlink.go) to pass the ID during increment/decrement operations and to explicitly call Destroy(). Additionally, a new test suite lookup_count_test.go was introduced to verify the behavior of the refactored lookupCount type. There are no review comments to address, and we have no further feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.86%. Comparing base (0673d19) to head (b5415c4).

Files with missing lines	Patch %	Lines
internal/fs/inode/base_dir.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5000      +/-   ##
==========================================
+ Coverage   83.81%   83.86%   +0.04%     
==========================================
  Files         174      174              
  Lines       21376    21376              
==========================================
+ Hits        17916    17926      +10     
+ Misses       2779     2773       -6     
+ Partials      681      677       -4     

Flag	Coverage Δ
unittests	83.86% <96.00%> (+0.04%)	⬆️

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