They — Use of the third person collective pronoun "they" — a disavowal of the project by the planners by the speaker.

. — The period makes this very short sentence declarative. There is no room for debate—the planners do what they do: plan and build. The process and curt and uncompromisable.

Gridded — to fit into a space is to enforce conformity; to turn "spaces" into boxes

Permutations of possibilities — ironic, as possibilities are open-ended, but in this case, closed off into fixed, acceptable sets.

The buildings are in… — Graceful, perhaps to the point of sterility. The imagery here is of dental braces: teeth are bound together and compelled into alignment, into meeting at desired points, but linking bridges.

Stop — irony: the period stops the line, but ossifies the declaration of the sentence: that there will be no stop to the building.

Sea draws back — reference to land reclamation

And the Skies surrender — the imagery here is that of the sky being blotted out by skyscrapers. these two bits of imagery above shows the tyrannical drive of the planners to the execution of their project. They compel even immutable nature to "draw back" and the "skies [to] surrender".

Blemishes — a word associated with dermatological flaw: the past is an ugly mark on the skin

Dental Dexterity — Consider the use of dental surgery as a conceit for the constant makeover of the cityscape; the demolishing of old buildings is compared to the extraction of the "useless blocks" of teeth. This is hardly a pleasant view of urban renewal.

Gleaming Gold — Shiny, yes, but very artificial and somewhat cold.

Wears — cosmetic; to be all dressed up; like dentures.

Perfect — perfect, yes, but cold, soulless.

Anaesthesia, amnesia, hypnosis — A very subtle line: note the increasing power of the words on the mind: from numbing, to forgetfulness, to brainwashing. The process of urban renewal starts painlessly enough, but ends in a whitewashed version of history.

Means — a pun: the have the ability, as well as the money.

So history is new — irony: history is by definition old, but here it is new made new.

Piling — imagine a dentist piling on your teeth, your jaw. Brutal. Bloody. Agonising.

Fossils of last century — the fossils refer to the buildings of the last century; the drilling mercilessly destroys them.

My — Note here the change in pronoun. See subsequent comment on the changes in pronoun in the poem.

Our — In the poem, the pronouns change from third-person collective, to first-person singular, to  first-person collective.
Significance: if the planners are monomaniacal in their drive towards urban destruction/renewal, the speaker is powerless to affect change. The outcome is the destruction of "our"--everyone's--historical heritage.

Tomorrow — The “past’s tomorrow” refers to the future of the past. The past can, as we saw, be destroyed--that is the planners’ plan for its tomorrow. The blueprint refers to the plan laid out for the destruction of the past. The speaker wishes he could “bleed poetry”: that he could use metaphorical blood to stain the blueprint, disrupt its overt neatness, to mess it up, to counter with poetry “the graces of mathematics”. His desire is fruitless however, as he is unable to wring even a drop of poetry/blood to “stain the blueprint of [the] past’s tomorrow.

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