refactor: golf only Padics/Hensel, NumberField/Norm

[yuanyi-350]

This PR selectively ports part of #38144 and only migrates the changes in:

• Mathlib/NumberTheory/Padics/Hensel.lean
• Mathlib/NumberTheory/NumberField/Norm.lean

Concretely, this PR:

• refactors Padics/Hensel by extracting the duplicated root-uniqueness argument into a shared private lemma a_soln_is_unique
• rewrites soln_unique to first transport the distance bound to soln via soln_deriv_norm, then close the proof by applying that shared uniqueness lemma, instead of redoing the full binomial-expansion argument inline
• refactors NumberField/Norm by moving isUnit_norm_of_isGalois to sit after dvd_norm and shortening its proof to a direct application of dvd_norm together with isUnit_of_dvd_unit

[github-actions]

PR summary 0e6b5bebd4

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

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You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).