Fix zero‑padding encoding for IPv4 (-zpp, -zpn) and remove duplicates

Summary

This PR fixes the IPv4 zero‑padding feature used by -zpp (zero‑pad permutations) and -zpn (max width) so that it:

• Generates the correct set of representations for a given -zpn value (no off‑by‑one).
• Removes duplicate encodings that were produced by the previous nested‑loop approach.
• Adds/updates tests (ip_test.go) that verify both correctness and absence of duplicates.

Tested on Windows and Linux; the changes are limited to the IP encoding helpers.

Motivation & linked issue

Fixes the behaviour reported in #642:

• Redundant results: four nested loops iterating over the same range produced many repeated IP strings.
• Incorrect result set: with padding=2 and ip=192.168.1.1, the result was always 192.168.1.1 instead of the expected permutations, e.g.
  192.168.1.1, 192.168.1.01, 192.168.01.1, 192.168.01.01.

This PR addresses both problems and aligns the feature with the CLI flags’ intent.

Issues:

• %#0Ns with strings does not match numeric zero‑padding semantics and produced surprising results.
• Nested loops used p? < padding with 0‑based widths, so -zpn N effectively meant widths 0..N-1 → off‑by‑one.
• Many duplicates due to formatting of identical widths across octets.

Improvements:

• Uses numeric formatting (%0*d) so zero‑padding is applied to integer octets exactly like users expect.
• -zpn N now truly permits widths 1..N. (No off‑by‑one.)
• Output is unique and deterministic.
• Guard rails: len(parts) != 4 and padding < 1 cases are handled safely.

CLI examples

-zpn 2 (with -zpp)

echo 127.0.0.1 | mapcidr -if 10 -zpn 2 -zpp -silent

-zpn 3 (with -zpp)

echo 127.0.0.1 | mapcidr -if 10 -zpn 3 -zpp -silent

Tests

• Updated TestIpEncodings in ip_test.go to assert:
  • No duplicates in the output (via set cardinality vs slice length).
  • Width coverage exactly matches 1..N for -zpn N.
  • FixedPad pads to exactly N digits per octet.

Example run:

go test -v -run TestIpEncodings .

Complexity & performance

• The generator performs N⁴ formatted writes for a single IPv4, which is acceptable for small N (the common use‑case is N<=3).
• Removing duplicates avoids extra work downstream (filtering/reporting), reducing overall runtime for typical pipelines.

Backward compatibility

• The change corrects the off‑by‑one in -zpn. If someone relied on the previous “N means N-1” behaviour, their result set will expand (never shrink), which is the expected and documented meaning of a “max width”. -zpn 1 remains identical to the original address (no leading zeros).

Checklist

• Fix off‑by‑one in -zpn
• Replace string padding with numeric %0*d formatting
• Deduplicate outputs
• Update/add tests (ip_test.go)
• Manual verification on Windows/Linux