feat(Topology/PartitionOfUnity): add pointwise_decomposition_finsum + companions

[anovickis]

Add three short lemmas to Mathlib/Topology/PartitionOfUnity.lean extending the existing PartitionOfUnity API:

• pointwise_decomposition_finsum — for f : X → ℝ and x ∈ s, f x = ∑ᶠ i, ρ i x · f x. This is the pointwise step that lifts to integral linearity in measure-theoretic PoU integration: ∫_s f dμ = ∑ᶠ i, ∫_s (ρ i · f) dμ.
• one_minus_sum_nonneg — 0 ≤ 1 - ∑ᶠ i, ρ i x. Direct rearrangement of the existing sum_le_one field; useful as a complement-mass remainder bound in chart-by-chart estimates.
• abs_le_one — |ρ i x| ≤ 1. Combines the existing nonneg and le_one; convenience for absolute-value bounds.

All three are short proofs using existing structure fields (sum_eq_one, sum_le_one, nonneg, le_one).

These came up while writing chart-by-chart Stokes-on-manifold estimates where one wants to decompose ∫_M f into chart-supported pieces using a partition of unity. The pointwise identity is the obvious first step; the other two are complementary algebraic bounds that show up in remainder estimates.

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PR summary aafc9e6204

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

+ abs_le_one
+ one_minus_sum_nonneg
+ pointwise_decomposition_finsum

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.