feat: object deletion support — Delete(), GroupWriter.DeleteAttribute(), allocator Free() (Issue #51)

[kolkov]

Summary

Implements full object deletion support per ADR-002, following the HDF5 C reference implementation.

Closes #51

New public API

// Delete an object (dataset, group, or link) from the file.
// Unlinks from parent, decrements refcount, cascade deletes when nlink=0.
fw.Delete("/path/to/object")

// Delete attribute from group (already existed on DatasetWriter).
group.DeleteAttribute("key")

What's implemented

Component	Description
GroupWriter.DeleteAttribute()	Delegates to existing deleteAttribute() — compact + dense storage
Allocator.Free(offset, size)	Free space tracking with best-fit reuse, adjacent block coalescing, EOF shrink
unlinkFromParent()	Reverse of linkToParent() — removes SNOD entry, rewrites B-tree v1
fw.Delete(path)	Public API: unlink + refcount decrement + cascade delete (contiguous + chunked data, group structures, OHDR)

C reference functions implemented

• H5Ldelete → fw.Delete() (unlink from parent)
• H5O_link(adjust=-1) → refcount decrement
• H5O_delete → cascade delete when nlink=0
• H5O__layout_delete → free contiguous data / walk chunked index
• H5MF_xfree → Allocator.Free() with FSM-style reuse

Changes

• delete_write.go — new file, fw.Delete() public API (+397)
• delete_write_test.go — 12 test functions (+480)
• group_write.go — GroupWriter.DeleteAttribute() + unlinkFromParent() (+182)
• internal/writer/allocator.go — Free(), coalescing, best-fit reuse (+190)
• internal/writer/allocator_free_test.go — 10 tests + benchmark (+256)
• group.go — minor lint fix (+1/-1)

Total: +1486/-20

Test plan

• Delete contiguous dataset — verify freed, surviving objects intact
• Delete first/middle/last dataset from multi-dataset file
• Delete all datasets from file
• Delete empty group
• Delete dataset with attributes
• Non-empty group rejection (error)
• Error cases: non-existent path, root "/", empty path
• Nested group bottom-up deletion
• Round-trip: create → delete → close → reopen → Walk()
• Allocator: free/reuse, coalescing, EOF shrink, best-fit, multi-cycle
• go test ./... — 100% pass
• golangci-lint run ./... — 0 issues
• go fmt clean

[codecov]

Codecov Report

❌ Patch coverage is 57.93103% with 122 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
delete_write.go	38.40%	78 Missing and 7 partials ⚠️
group_write.go	61.17%	19 Missing and 14 partials ⚠️
internal/writer/allocator.go	93.93%	2 Missing and 2 partials ⚠️

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