fix: name the storage delete hooks as the core declares them

[benjaminbruun]

The port defined its delete and create-dir hooks as ss_remove/ss_rmdir/ss_mkdir, names nothing calls, so the core's weak ss_delete_file/ss_delete_dir/ss_create_dir stayed undefined; on the nRF91 the linker turns those calls into nop.w and DELETE FILE answered 9000 having deleted nothing (on native_sim it is a NULL call). This renames the three functions to the declared names, no logic change, and adds two tests/fs cases covering delete of a middle and of a buffered last entry, the not-found path, flush-after-delete and remount. No exposure with the shipped template: its EF.ARR grants no DELETE FILE access, so the command is refused with 6982 before storage is reached. The test bodies are black-box and also pass on #197's swap-with-last rewrite; removing either its slot re-point or its hole-closing copy makes them fail (checked: 13/14 in both cases, only the new test fails). Footprint: rename only; nrf9151dk/ns app image 150,124 B flash / 74,040 B RAM, the delta versus master is the three restored branch instructions.

[github-actions]

Firmware footprint delta — nrf9151dk/nrf9151/ns

softsim_external_profile app image, from the ELF program headers (flash = load image incl. initialized data, RAM = static SRAM use incl. .bss/noinit):

Memory	base	PR	Δ
Flash	146.1 KiB	146.2 KiB	+96 B
RAM	72.3 KiB	72.3 KiB	+0 B

Section size changes (from the section headers)

     +54 B  .strtab              (65698 -> 65752)
    +144 B  .symtab              (82080 -> 82224)
     +88 B  text                 (111776 -> 111864)

Top symbol changes (10 largest, from the symbol tables)

     +72 B  ss_delete_file
      +8 B  nvs_delete
      +4 B  ss_delete_dir
      +4 B  ss_create_dir

_{Soft report — informational only. Base 418e66207ab626f4a312fdcf6d5656eb97d5c712 vs PR head.}